sts means sent to streaks. You see sts in Snapchat were people compete to have the most streaks. Now is you get a sts you know they have or want a streak with you.
A, typically, gaming abbreviation meaning "Sucks to Suck" used whenever someone does something unfortunate such as die in a video game. Could also be used in real life situations if someone does something bad like fall down.
Barton: I just died to fall damage!
Fowensic: Wow, StS, bud