Now, Google+ is going away (April 2, 2019), and Blogger is the only social networking left at Google. It's a Blog, and not *really* social networking, just like LiveJournal, WordPress, and most of the other first-gen style.
MySpace converted to a musician hub, but it not really a general social networking hub. The layout is not conducive to friends-of-friends on anything other than artistry.
Tumbler, Instagram, Imgur, Flickr, etc are about photo sharing, and are not really great for media sharing or non-image social networking.
Lots of exercise, fitness, and health networks exist, but they do not really support non-fitness socializing.
Orkutt, Friendster, sixdegrees and others are gone. FaceBook is still a big thing, but it's pretty heavily based on meme propagation, privacy scraping, and is full of a lot of false information, propaganda, etc.
Twitter is still pretty big, though it is tailored towards 160 character messages, is kind of spammy, and suffers a very strong mob warfare mentality.
MetaFilter and Reddit are sort of news aggregators, and community discussions. There's not really friend groups, event calendars, etc; however, they are pretty much the best options available lately for group social.
For messaging, twitter sort of bridges that gap in a weird way that people like, but not me, mostly due to layout of the app, and filtering issues with the design. SMS is pretty limited the other way, and has poor privacy controls. SnapChat is all about photos, and is very ephemeral. It's pure vanity/narcissism. GroupMe has security issues.
Messenger, Google Hangouts, WhatsApp, Discord, and Slack are pretty much the best options. Discord is very privacy oriented, and good for games, but hard to connect to people you don't already have a connection with. Slack is very group/project oriented, and does not have a global DM system connecting you between projects; each site is its own thing. WhatsApp is a cheap SMS replacement for international users, and tends to not always notify on new messages. Messenger being great for people who use Facebook. It does groups, individual, and has decent privacy. Google Hangouts suffers the Google issue of "how do we monetize this", and has been renamed, reservered, re-APIed, etc a few times. It still works, and it's well aligned for people who use gmail and google calendar.
Pretty much, if it exists, I'll be "xaminmo" there. I've outgrown the name, but it's reliably available. It helps me stay me. I'm mostly on Reddit, Messenger, and gmail/hangouts though. Anywhere else, I might not notice messages.