It can be difficult to trust somebody. However it is not a good idea to give yourself undue stress.
When I was younger I used to have MAJOR trust issues. The way I got over it was assuming positive things, got example when she's out with her friends assume that she isn't and will not cheat. Trusting is basically assuming the positive, assuming the positive is easy.
Checking her facebook will only feed paranoia and negative assumptions. There are photos of me with girls on facebook who are simply very good friends of mine, but a paranoid mind would run away with such photos baselessly.
So basically:
Don't talk to her about it, never check her phone/facebook and never make baseless accusations.
Assume that she hasn't and won't cheat.
HOWEVER if there is a red flag for whoreishness, go back to the packed crowd and look for somebody else
