I'm not sure if it would be considered social by most, but I enjoy using reddit. Its a great way for me to keep up to date on global matters daily.
I use Facebook. I usually have in on the majority of the day and check it every now and then to see what it new. I think it is a great way to keep in contact with family and friends that I wouldn't otherwise do. I have, also, reconnected with a lot of old friends which has been nice. I do use twitter but not as often as I use Facebook.
Well I am not a big Twitter fan, but I seem to be on it a lot. Then Facebook, which I thought of closing, but then again I do keep in contact with some family member's there so it stay's open for now.
I am not a fan of Reddit (seems confusing and disorganized to me), but you have probably seen the recent turmoil instigated by Ellen Pao. As I understand it, editors closed sub-forums in revolt. A new news and social site, Voat, is attracting attention from disgruntled Reddit users. Perhaps it may be of interest to you. http://www.marketwatch.com/story/voat-creator-speaks-on-reddit-refugees-plans-for-future-2015-07-10
FWIW, more on Reddit here. http://gawker.com/no-one-wants-to-admit-it-but-reddit-cant-be-saved-1717577917
My personal favorite social media website is Twitter. I love being able to give random thoughts or speak on the latest news. The search feature and hashtags are awesome because they allow me to get other people's opinions, even if I don't personally follow them, by searching any topic. Twitter keeps me up-to-date on the latest news and entertainment without having to go to different websites for certain types of news. I log on, at least, every couple of hours and use the "while you were away" feature to see anything that I may have missed.
I use Facebook and Instagram. I usually keep in contact with childhood friends on Facebook. I enjoy being on Instagram because I love looking at pictures.
Facebook is a real no-brainer but if you really want to get connected professionally then LinkedIn it is. Twitter and instagram is also great for micro-blogging and photo sharing business or personal use-wise.