I bet that you're quite computer literal yourself (hard to suppose otherwise since we're talking through HN) so as to be quite cautious as to how much of yourself you'll expose on the net. Unfortunately most people aren't plus they are quite good at documenting their lives (photos,videos,writings, etc) and linking in with people they know and share data and the like.
That's "tons of personal data" - it's not an absolutist firework but it's the main reason that gives FB such a high market value. AFAIK FB is considered a gold-mine in terms of highly targeted advertising and marketing that's because it hosts "tons of personal data" of high granularity and of high relevance to the real identities of the people uploading it. Otherwise wordpress for example would be considered of at least equal value with FB.