1.) Used in text messaging, it simply means to be horny.
2.) It is used as a type of facial expression in a text message, like a smiley face is used to show happy and a frown face is used to show sad, @~@ expresses the horny affection.
Chase: Hey guy what're you doing.
Jables: Watching some porn.
Chase: Wow, that kinda makes me a little @~@.
Jables: I've been @~@ all day, need to go to my gf's.