Teenager Help, please!

Foster parents with teenagers: where do you draw the line? At what point do you "give up" on a child/teenager? I don't really mean "give up" because that sounds so harsh, but at what point do you ask for the child to be removed from your home? We've had a rough couple of days. Teen ran away, did alllll things you'd imagine and found out that she's been doing lots of stuff for a while behind our backs. I know she needs help and I thought we were doing that for her (along with therapists, etc), but apparently not. She is now a danger to herself (making reallllly bad choices) and probably a danger to our family (I have a 2 yr old and a 4 yr old) too. Danger meaning, probably too many of the "wrong kind of people" now know where we live. She's 17. I'm lost. I reallly want to be there for her. I want to help her. (so does my husband) But I'm afraid she is going to end up in a ditch one of these days/nights. HELP!