There are various social media sites, but it's not necessary that for all niche all the social media platform will work equally. Like LinkedIn is more of B2B and Facebook is of B2C, so here target audience plays an important role to decide for which social platform one should go.
Facebook
YouTube
Instagram
Twitter
Reddit
Vine
Ask.fm
Tumblr
Flickr
Google+
LinkedIn
Pinterest
VK
Classmates
MeetUp
are most popular social media sites.