Gregaaron - there is a weird but very real human desire to not go in the way of tzniyus. This includes getting involved in the affairs of others and be privy to information that is otherwise private. This is, btw, the desire that "reality TV" plays on and is the base reason why it's appealing to people.
Facebook, provides the means for people to show the world parts of their lives that other people would not otherwise show. Nobody needs to see you drinking coffee with your wife in the patio, nobody needs to hear about what you had for breakfast, or what shoes you bought. It's not that these things are taboo, but the concept of putting a notification of all your daily activities on the town bulletin board is the antithesis of living a humble and tzniyus life. It ruins our sensitivities and wears away our refined appreciation for what's private and what isn't.
Friendships which are created and fostered through human interaction are put on the same plane as mere acquaintances through the convenience of virtual messaging. Thus, social skills and relationships are weakened.
People spend hours sifting through friends' photo albums, messages, updates, etc. The whole thing is destructive in so many ways. The Facebook corporation spends plenty of research and money figuring out ways to get their users to spend more time online, and it works.