Here is the situation:
I was waiting at the bus stop along with a blonde girl that I have layed eyes upon for a while and her friends.
I was nervous because the upper two buttons of my pants got somehoe ripped of and I had trouble keeping them up, despite that I still managed to start a conversation.
Once the bus arrived we all got in the bus and while I was bussy looking my card to stamp the bus already started to drive and I not only feel but also pulled my pants down by accident...
There I was lying on the ground right in front of the girls with my butt in the air, giving them excellent expossure of my white underpants.
Before I could even realise what happened I could already hear them laughing and tacking pictures with their phones, I really don't know how to face them again after this...
Been there, done that (similar). I think you'll find most have (hence their gleeful relief when someone else does it).
How to face them again: Show you acknowledge the fact you're only human, same as them, and beat them at making the most amusing comments over it. Were it me, I'd say something like,
[a] 'You don't have to tell *me* I have a fascinating butt. I already know. Hence whoops-lost my trousers as enabled you to take lots of snaps. Aim, fire, bullseye! regarding my cunning plan to gain widespread publicity. Please direct all enquiries from any talent scouts looking for Film Body Doubles to me. Cheers. :-p'.
[b] 'It's a new and cunning advertising campaign by [make of underpants] and I'm one of their undercover campaign models. I get commission for each and every exposure, not just every purchase, so, on behalf of myself and my bank manager: please keep up the good work re circulating it! :-p'.
[c] 'Well, my trousers were perfectly secure *before* I got on the bus, so obviously you gals should from now on be more careful and responsible with your telepathic powers of wishful thinking, shouldn't you.
[d] 'A mate bet me £50 I wouldn't dare flash my butt at you. You carry on taking the p*ss and I'll enjoy spending my cash.'
My vote is C or D.
Hope that helps. Otherwise, just don't react and wait until you're very soon yesterday's news.