For now I'd suggest that you go through all of your social networking profiles and ensure the privacy options are all ticked so that your profiles aren't viewable by somebody you don't know.
Moderate your teens usage and only let them on for a certain time of each day maybe? That way you know they're not spending too much time on it.
Maybe think about settin up privacy options on the browser as well?
I'm probably not the best person to answer this but I'm sure you'll get somebody else commenting soon so don't worry.
Perhaps check on Google for some free software to protect you from spyware and malware, but make sure it's not a virus first etc. Check up reviews and stuff to ensure it's all legit.
AVG anti-virus is a good one I think.