internetaverted

Someone who is seemingly emotionless in person, but always posts online with a lot of excitement (a lot of exclamation marks, all caps, etc.)

Jennifer, being very internetaverted, sat quietly in Starbucks, eyes glued to her phone, without so much as a polite smile on her face while posting to Facebook, "OMG!!! HE PROPOSED TO ME!!! I'M GETTING MARRIED, EVERYBODY!!!!!! AAAAAAAAHHHHH!!!!!!!!"