Each site is has different aspects and you may find one suits you better than others, below is a quick overview of some of the most popular sites.
Social media sites
Facebook
One of the original sites, with tons of features for keeping in touch, sharing content selling items etc. It's free but does track everything you (including outside Facebook) do to sell your info to advertisers.
Instagram
Owned by Facebook with similar features but focused more on sharing images and now videos. Uses the same process as Facebook for selling your behaviour to other companies.
Snapchat
Great for keeping in touch and giving little updates to your friends and family. Is rather fun with great filters for sharing funny moments. Messages disappear once read unless you decide to keep them.
Twitter
Bite sized updates you can send to everyone who follows you. Use dmostly for sharing opinions and news.
LinkedIn
More career focused site used for job hunting and career progression. Has since expanded into a learning platform.