The problem with Facebook is that it holds way too much personal information about a person - phone numbers, emails, a person's likes/dislikes, hometown, current location, etc, and because society has been 'programmed' to share so much about themselves, no thanks to social networks like Facebook that promotes building your 'profile'.
In fact, strip away all that personal information and have people share their thoughts and their dinner photos and what you get is just Twitter, Instagram, Snapchat or a blogging platform.
A decentralized alternative to Facebook will not solve the problems Facebook has because in the end even if you own the private key to your own data it's up to you if you want to share your data with someone or an app, and once you've done that to a malicious party, your social network is compromised. And as some have pointed out in this thread, a decentralized and open source alternative would be worse.
In the end, it's up to the individual to be smart about what to share and what not to share, and reveal as little about themselves as possible rather than parade it all out to the world or to their 'friends' list. All it takes is for someone not too technical to download a hacked 'client update' to their decentralized Facebook alternative to have everything they thought to be secure be leaked out.
An alternative that promises to be more secure than what it's replacing is just asking for more complacency. I'm sure we thought Facebook was extremely secure at some point, so why not share everything?