Simultaneously having an online IM chat with several people while also sitting within conversation distance of said people in real life, especially on Facebook.
Random guy: Hey, what are you guys all laughing about?
Chatting guy: We're all multichatting on Facebook.
(gets a message)
Chatting guy: LOL, he totally does!
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.