We just can't predict that which will be on the number one position among them. The facebook and twitter are doing great but never now Google+ and Linked can also be great or any new one can also get on the floor to compete all of them.
Our generation's sleeping schedule depends on the percentage of battery remaining in the phone because we are using facebook on mobiles also. I will go for Facebook
Honestly I don't think Facebook would ever get off the #1 spot in social media as Facebook has been for several years and will be ever since it thumped MySpace who were #1 several years ago. By I mean for several years, Facebook will be #1 but forever? That's real hard question because it could be a possible considering that nobody thought MySpace would move off the #1 and Facebook moved in #1 spot. However I can give you two social media websites that is fighting for #1 would be Facebook vs Twitter. Google+ is still too far behind and the news has started resurfacing that Google might to discontinue Google+.