I agree with a lot of what Colleen's said, but I think the DD has already used bad judgement here:
I've seen pics on social media. not a lot..only 2 but pics w/them doing shots and then just a group hanging out photo w/alcohol in it. She is not aware i have seen the pics.
A picture like that floating around on social media can cause big problems for students today. Our HS is very strict about it, there is a zero tolerance policy for drugs and alcohol. As Christine said, one wouldn't have even have to be drinking - just being in the presence of alcohol is enough, as far as the rules go (although it sounds like in this case, the DD was actually doing shots). Some of my kids' HS classmates had a drinking incident that made the news and some scholarship and college opportunities were ruined.
It is no longer like it was "when we were growing up"!
And honestly, one of my biggest concerns would be driving. Does she have access to a car or do her friends that she's with? Driving a car comes with big responsibility. Only you can determine that.
At this stage, I would have a heart to heart with DD and talk to her about these things. I wouldn't involve the other parents unless it continues to be a problem.
Personally, I'd be concerned.
Just to add, sort of like what Minnesota was saying, I think that parental attitudes here are determined a lot by the parents' own experiences and habits. I think it's wise to take a look, when dealing with problems like this, at what type of behavior we're modeling for our own children. It's a little hard to tell kids not to drink when we're drinking heavily ourselves every night. Modeling responsible drinking probably isn't a bad thing, since they are going to have exposure and pressure. But I think at 16 it's still a little young. As another poster mentioned, it gets harder at 18-19.