It's just a username. As implemented the systems only require a username. It's also not even that, it's a temporal identifier, as faces change, sometimes in ways that we all expect and sometimes, not. To the extent that we've even performed facial transplants in response to some of these cases.
If biometrics were going to work, we'd be using fingerprints already. For all the same reasons we don't use fingerprints, we won't be able to use facial identification.