You're their parent, not their friend. Your job is to protect them, sometimes from themselves. Every teen thinks they can handle anything that comes their way and resents any authority that tells them different. Until it hits the fan. Then it's "Daddy, fix this."
Lay down clear rules and let them know you'll be checking. And follow through. One empty threat and you've lost all credibility.
I know the popular and "progressive" thing is to "trust" and allow them their "privacy". IMO, that's lazy and irresponsible. And those parents are always so shocked when they find out their little darling has been selling drugs or having an online affair with a 48 year old married man.
You may have the best kids in the world, but their friends aren't. Or their friends' friends. And things can spiral pretty damn quickly.
Monitoring their electronic social life is no different than monitoring their use of the family car. Who, where, when.
I'm sure I'll get lots of flack for my opinion; so be it. But ask any cop or judge who deals with kids what the number one problem is.