When I was a kid, long before Facebook, my mom would frequently receive an anonymous chain letter in the mail.
The letter would have some dramatic emotional appeal, or it would claim to be raising money for a poor family in the church, or it would promise pyramid riches, or it might have some mystical mumbo jumbo warning about breaking the chain.
But in the end, the stories about heartbreak or helping people or mysterious calamities were complete horseshit. Their only purpose was to convince my mom to copy the letter and send it to 12 other people, thus expanding the base of the pyramid for the benefit of whoever started the chain letter.
Facebook is the modern chain letter.
Just like chain letters, Facebook has lots of horseshit stories about connecting people, with implied mysterious social consequences for not participating.
But underneath all that, the real purpose of Facebook is to amplify social pressure to get people to expand the base of the pyramid for Facebook's benefit.
Guilt is a strong motivator, as least for me, so granting someone that access makes it a lot more difficult to set appropriate boundaries.