- Klaviyo syncs lists and segments to Meta as custom audiences, one-to-one, with changes added or removed every hour.
- Meta needs at least 100 profiles to build a custom audience, and the synced audience usually won't match your segment size exactly.
- The first sync I'd set up is an exclusion: stop paying to "acquire" people who already bought.
- Ads are rented, your list is owned. Syncing is how you make the list you own make your rented traffic cheaper.
How does Klaviyo's Facebook audience sync work?
Here's what Klaviyo's own help center says about it.
- What syncs. You can sync lists and segments to Meta as custom audiences. Each sync is one-to-one: one Klaviyo list or segment to one Meta custom audience.
- How often. As people are added to or removed from your list or segment, they're added to or removed from the Meta audience on an hourly basis.
- Who matches. Only email addresses in Klaviyo that are tied to a Meta login get synced. So your audience in Meta will usually be smaller than the segment in Klaviyo.
- Minimum size. Meta requires at least 100 profiles to create a custom audience.
- Delay. Custom audiences can take 24 to 48 hours to update on Meta's side, because Meta has to accept and process the profiles.
- Setup. You connect with the personal Facebook account tied to your Meta business account, accept Meta's terms if asked, and pick the ad account you want to use. Then you turn a sync on or off from the Lists & Segments tab.
That's the whole mechanism. The hard part isn't turning it on. It's deciding which segments are worth syncing and what you do with them once they're in Meta.
Use 1: Keep customers out of your acquisition ads
If you're running prospecting ads to find new customers, every dollar spent showing those ads to people who already bought is a dollar spent on nobody new. That's where I'd start.
Here's how I'd set it up:
- In Klaviyo, build a segment of everyone who has placed an order at least once.
- Sync it to Meta.
- In Ads Manager, exclude that audience from your acquisition ad sets.
Because the sync runs hourly, a new buyer moves into that segment and out of your prospecting ads on its own, give or take Meta's processing delay. Your existing customers are already getting your post-purchase emails and reorder reminders. They don't need a "first order" ad too.
One caution. Only people whose email matches a Meta login get excluded, so this cuts the waste, it doesn't erase it. Don't read your Meta audience size as your customer count.
Use 2: Build lookalikes from your best customers, not your whole list
Klaviyo's help center suggests syncing a VIP list or segment and building a lookalike audience from it in Meta, so you reach new people who resemble your best customers. I agree with the idea. The seed matters more than the setting.
If you seed a lookalike with your entire list, you're telling Meta to find more people like everyone who ever filled out a pop-up, including the ones who never bought and never will. Seed it with people who bought more than once, or your top spenders, and you're asking for something specific.
For a consumable brand, my pick is customers with two or more orders, or the VIP list I build for clients by spend, with the threshold set per brand. Those are the people who got through the hardest part: the second order. That's the whole point of retention, and it's the line on my homepage: make the second order automatic.
Use 3: Retarget engaged people who haven't bought
The third use is people who clearly like you but haven't ordered. Think: subscribed in the last 60 days, clicked an email in the last 30, no order yet.
Your welcome flow and abandonment flows are already working on these people by email. A Meta audience gives them a second place to see you. Keep the window recent so you're not paying to chase people who signed up two years ago and went cold.
Which Klaviyo segments should I sync to Meta?
Here's where I'd start. Build these as segments, not static lists, so they update on their own.
| Segment in Klaviyo | What to do with it in Meta |
|---|---|
| Everyone who has placed an order | Exclude from acquisition ads |
| Customers with 2+ orders, or your top spenders | Seed for a lookalike audience |
| Engaged non-buyers (recent sign-ups or clickers, no order) | Retargeting audience |
| Lapsed customers past their usual reorder window | Optional winback audience, alongside your winback flow |
| Unsubscribed or suppressed profiles | Exclude, if you don't want to advertise to people who opted out of email |
Each of these needs at least 100 matched profiles to work in Meta. If you're a smaller brand, start with the customer exclusion, since that segment is usually the biggest useful one you've got.
What mistakes should I avoid?
- Syncing your whole list and calling it done. It's the least specific audience you own.
- Building audiences that are too small. Under 100 profiles and Meta won't build it. Wider windows help.
- Expecting the numbers to match. Klaviyo says your audience size may not match your segment size. That's normal.
- Forgetting the delay. If you launch a promo tomorrow, set up the audiences today. Meta can take 24 to 48 hours.
- Letting segments rot. If your "VIP" definition was set three years ago when your average order was half what it is now, update it.
- Skipping the privacy check. Make sure your privacy policy covers using customer emails for advertising. Ask whoever handles your legal stuff, not me.
Why bother? Ads are rented, your list is owned
Your Meta audience lives on Meta's terms. Prices change, targeting changes, and you don't control any of it. Your list is yours.
A subscription wellness brand I work with had a month where store revenue fell 37% and new customers dropped from 1,657 to 223. In that same month, email's share of revenue went from 41% to 53%. The list kept paying when new-customer volume fell off a cliff.
Syncing doesn't replace that. It connects the two. Your owned list tells your rented ads who to skip, who to look for and who to remind. If you want the flip side, how email carries more weight when ads get expensive, see email marketing when ad costs rise.
FAQ
Can Klaviyo sync to Facebook custom audiences automatically?
Yes. Once you connect Meta and turn on a sync for a list or segment, Klaviyo adds and removes people from the matching custom audience on an hourly basis. Meta can take 24 to 48 hours to process the changes.
Why is my Meta audience smaller than my Klaviyo segment?
Klaviyo only syncs email addresses tied to a Meta login, so anyone who isn't on Meta, or signed up with a different email, won't match. Klaviyo's help center says the sizes may not match exactly.
What's the minimum audience size for a Klaviyo sync to Meta?
Meta requires at least 100 profiles to create a custom audience. If a segment is smaller than that, widen the time window or combine it with a similar group.
Should I exclude existing customers from all my Meta ads?
From acquisition ads, yes, that's where I'd start. You may still want separate ads aimed at customers, like a launch or a restock. Just keep them in their own ad sets so you can see what each one costs.
Do I need an agency to set this up?
No. The sync itself takes a few minutes. The value is in picking the right segments, which is easier once your Klaviyo segments are built well. Start with email segmentation for ecommerce.
Founder of Bardown Digital, a Chicago-based Klaviyo email and retention agency for Shopify brands. He's worked with brands doing $500K to $100M a year, with $100M+ in email-attributed revenue across them.