Facebook says 20.
From experience, I’ve launched campaigns with retargeting components and seen single impressions in the retarget audience very soon after starting both campaigns.
I can’t say for sure that there were less than 20 in the audience to retarget, but it sure looked like it.
When you build the audience, you will see a note from Facebook that the audience is too small or populating. You can still start the retargeting ads running. he ad will show as soon as someone is available.
I’ve seen people say the minimum audience is 1K or 1oo, neither is accurate.
You can’t upload an audience of 1 person, though like you could a few years ago. (You’re not supposed to be able too, anyway).
If you are building an audience for retargeting, just let the ad run as soon as you can. If you budget is small, its not an issue. Over time you will have to keep an eye on ad frequency and make sure not to burn out the audience.
When you run standard ads (not retargeting), Facebook will show that the audience is “too small” with anything less than 1000 people. Run the ad anyway if that is your audience. See if it gets impressions. If it does, then the audience wasn’t really too small.
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