My guess would be Instagram.

WhatsApp (owned by Facebook) 1,500,000,000
Messenger (owned by Facebook) 1,300,000,000
WeChat 989,000,000
Instagram (owned by Facebook) 800,000,000

Well normally Peple use , Instagram, twitter and linkedin. these are also best social networking sites..

According to my Experience after Facebook top networking sites is twitter and linkedin

Twitter is known as the real-time, public microblogging network where news breaks first. Most users love it for its short message limit (now 280 characters) and unfiltered feed that showed them absolutely everything in the form of tweets.

Twitter has changed dramatically over the years, and today it's criticized a lot for going the way of looking and functioning almost exactly like Facebook. Besides Twitter Card integration, which now makes it easy to share all sorts of multimedia content in tweets, you can expect to see algorithmic timelines coming to Twitter as well.

Twitter
Instagram
Reddit
myspace
linkidin
plurk etc...

1 – Facebook. This is easily the largest social networking site in the world and one of the most widely used. ...
2 – Linkedin
3 - WhatsApp

Go for instagram, according to reports although Facebook have more users then Instagram but instagram have more active users then Facebook

Instagram is most important in Social Media Marketing.
Twitter is also important but Instagram have a unique postion in Social Media Marketing.

Twitter is much better than facebook I think. Also IG.

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