My favorite (paraphrased) Cory Doctorow quote pays dividends here:

"Facebook started as a way to nonconsensually rate the fuckability of Harvard graduates, and it only got worse from there."

There was a brief period where Facebook was cool enough to give it public credibility, buoyed by the fact that nobody knew who Mark Zuckerberg really was yet. After that point Facebook was big enough to just buy more credibility by buying more successful competition.