I tend to not worry about privacy on Facebook since I use it as if it's a publishing outlet. Basically everything I put on Facebook is stuff I would broadcast in the newspaper, so to speak, if the newspaper were willing to publish it.
Political manipulation is a big problem, but I think we can immunize ourselves from it. The responsibility lies on us as readers to be careful what we react to. As readers of any media, we should try to not fall for emotional and divisive things. We should give our reactions time to due fact checking and so forth. As the old saying goes, don't believe everything you read on the internet, or for that matter, the mainstream media as well.
Little folks, like me, don't have columns in newspapers, TV shows, or Twitter accounts that start national conversations every time they utter something; like in the case of Donald Trump. Facebook has become a public forum for me and it has the inertia of friends which provide feedback to my writing that is not available other places. I use it as a publishing outlet so I'm basically not looking to it for privacy.
I often float ideas on my Facebook wall and then put some of the things on this blog. The things that seem to play well there given feedback and so forth.
As for other social networking sites, I use Google Plus, but practically never hear from anyone on it. It can be a lonely world out there. I use Google Plus because my blog has a tab that allows me to share posts there. A few friends are signed into my Google Plus account that I seldom hear from. Also it does have one feature that I don't think Facebook has. It allows one to set the privacy all the way open to the entire web. One does not even need to be a Google Plus user to see the posts (if one uses that setting) so I guess they can even be cataloged in Google. I think, with Facebook, the wall can't be visible unless readers are logged into Facebook; except for fan pages.
There's also a service called Ello. Several years ago, I signed up for it, but haven't been motivated to look at it ever since. I'll admit I am part of the inertia issue that keeps Facebook on top. Only so many hours in the day. Ello keeps sending promo mail to my email address, but I get so much promotional and impersonal email that I don't open most of my email unless I think it's from a real person. Maybe I should give Ello more of a try. At least to try and turn off their frequent promo mailings if nothing else. Facebook is kind of the go to for real people. Inertia at work.