Can I show Facebook ads to specific leads?

In general, this is a difficult question. If someone answers you that it's easy and works somewhere in one click - don't be fooled.
Technically, from the OneBox side, these are actions at the stages of the business process "Add contact to facebook'a remarketing list" (the name is approximate, I don't remember exactly). This action is currently not in the client boxes (in releases), but it is in the intermediate branches of the developers. I'll explain why below.
Then the fuss begins on the facebook side, namely:
- create remarketing lists
- set up ads for them
- and hell: create an app in developers.facebook.com, give this app access to your remarketing lists, pass basic verification.
So, in my practice, out of 4 attempts to create a facebook app and give it access to a combat advertising account facebook ads so that it can start / stop ads, change remarketing lists - it really happened 1 time, it worked for 4 weeks, then facebook banned me "by not very clear reasons" (I can guess, but I don't know for sure).
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To answer your question more specifically:
Yes, you can do it with OneBox. But, you must be prepared to personally mess around with facebook and set up ads. OneBox only manages remarketing lists here.
There is an alternative option - to use something like zapier, but it is expensive and does not work better (I checked it, I spent the loot).