It’s more private than Facebook, less private than it should be. Yes, it’s closed-source. You can create groups, join groups, add friends, private chat, make posts, etc. Frankly, I liked it better than Diaspora, Mastodon and other open source social networks despite the obvious shortcomings with it being closed-source. Your mileage may vary, of course. Fair warning, though: There does seem to be a rather large population of the MAGA-hat wearing, QAnon Qultists there (because they were kicked off of Twitter and Facebook), so go in with that understanding.