I think that this is driven by joint FB-IG campaigns. Before this change, if they believed that it was the same user (presumably through some kind of model), th

I think that this is driven by joint FB-IG campaigns. Before this change, if they believed that it was the same user (presumably through some kind of model), they counted a pair of accounts as 1 person. After this change, they would count them as 2, given that the accounts aren't officially linked. To be fair, I suspect that an awful lot of accounts (at least in my circle) are linked as I see a bunch of posts on FB which come from IG. Overall, this seems pretty sensible. All other things being equal, you'll see an increase in actions per unique people, but given that most advertisers use CPA to track performance, this shouldn't really be a big deal.