I would hazard a guess that the facial recognition will limit the search scope to people associated (to some degree) with your friends account and some threshold of metrics gathered from the image. I doubt it is using a broad search.

With billions of accounts, the false positive rate of facial recognition when matching against every account would likely make the result difficult to use. Even limiting to a single country like UK the number could be extremely large.

Let say there is a 0.5% false positive rate and some amount of false negatives. With 40 million users, that would be 200 000 false positives.

The only explanation for this comment is you never used reverse image search by Google or yandex before it was nerfed or you'd know this is super plausible to find direct hits without many false positives.