The Big Five has a lot of external validity (as do the regression estimates of Big Five, if you read the paper), so that's fine. If things like Conscientiousness really do predict income, longevity, etc, then even if they're a 'self-judgment', why is that a problem.

These self judgments also tend to be correlated with judgments made by friends, family, spouse, therapist, and with personality tests. In other words, even though there are differences between the groups, people are relatively self-aware.

I'd be interested to see here if friends in face-to-face contexts perform similarly to the Facebook context.

I don't think it's self judgement, they are asked to take a personality test which is a bunch of questions. The personality traits are literally defined as the way people answer the questions on the test.

It's not self-judgements, it's people expressing a like for something completely unrelated. Nobody "Like"d Nikki Minaj because they were hoping to show themselves as Outgoing and Active.