They want to be accepted - looking at other teenagers using them makes them feel like that’s what they’re supposed to do. They’re posting various parts of their life online and they’re seeking for acceptance of their peers through the likes, great comments and retweets / shares.
They’re following celebrities - everyone has it’s fan, personal favorite, whether it’s singer, an actor, fashion blogger etc. They feel powerful being involved in various aspects of lives of their icons.
It’s easier for them to connect with their friends - some teenagers are shy and not that great at live discussion, but they’re more comfortable through messages and posts. You can wait for few seconds before you say something, noone is there to see your red face or hear crazy laughter and you’re pretty much relaxed.
They don’t know what to do beside that - harsh truth is that most of teenagers on social networks are typically bored, scrolling through their feed without particular goal. They’re completely aware of wasting their time, but there’s nothing interesting to do at that moment beside wasting precious time.