A person that is well known on Twitter, and may be a professional in the entertainment industry, but you've never heard of them outside of Twitter. TV writers and stand-up comedians that you've never heard of, sometimes just regular people that are funny enough to build a following.
"That Jokie Jokerson is hilarious!"
"I don't know who that is."
"Really? Man, he's huge on Twitter!"
Oh, a celebritwee.