Linkedin: online resume
twitter: for people to broadcast clever witticisms, suck up to their professional contacts, complain to brands. Very front line of "personal brand." Seems like people have two twitters pretty often- one for random chat about whatever and one "professional" one to push their career agendas.
facebook: bragbook for social striving/status updates of vacations, jobs, new cars, cute kids, flattering photos. Also to cheer on and suck up to friends and family/like their content. Getting more and more annoying lately with political posts. Being able to block people from your newsfeed has helped with this immensely.
instagram: more personal photos than facebook, less likely to be "friends" with your boss etc, also less likely to be attached to your real name, so easier to showcase your real personality/quirks. More artsy than facebook.
snapchat: original video content, people are more "real" than on facebook because there is less perceived risk of people they know irl or professionally finding them and seeing all their doofy posts. Here I am riding my bike, here I am at the beach making a silly face, etc. People feel more free and less worried about it hurting their careers. More fun than facebook with the facial editing tools and stickers which seem really neat.
periscope: dunno, never used it but heard it's the hip new thing
about.me : I still don't understand this one. Guess it's supposed to be the landing page for all your other social medias?
medium.com: starting to pick up lately, not just a place to write Open Letters to Whoever anymore! Extension of personal brand for writers.
Path: dead
Google plus: annoying, stupid
Reddit: enjoy the anonymity, so refreshing after being forced to use the "real you" constantly on facebook etc. Surprisingly intelligent discussion and relevant content in the subs. Was actually recruited for a fantastic job off of my reddit comment history of all things!
hackernews: kinda new, still learning the ropes around here but I've been loving the intelligent discussions and learning a lot.
quora: Place to show off your smarts and push your personal brand. I am actually really annoyed about this because I used to answer and ask a lot of great questions on quora about embarrassing stuff (LOTR, Star trek, whatever) and then all my coworkers and professional contacts started following on me and now I feel very constrained on there.
any other thoughts?