If you already have a following you don't have to self promote that hard. Still though plenty of comics post one liners on twitter and facebook and it gets them followers.

I know slashdot views everything on social networks as trivial but social networks need content creators like comics to function. New content keeps people tuned in and checking for updates. Content creators post content for self promotion. Comics do it to raise their profile and get people to physical shows. If someone can just plagiarize it and not attribute it to the comic to the point where they drown you out this defeats the whole reason they post it. I'm a no-name comic (and a software developer) but even I had a joke stolen by some aggregator, He got like 10k+ likes on it (it was about the crash of a truck full of ramen noodles). Of course I had less than 100 on my post because he had a bigger following. But if he had just shared my post instead of pretending to take credit for it I could have used those likes and publicity to get a couple dozen more people at one of my shows and we'd all be happy. It makes me not want to bother posting my jokes. Now I'm not important but if more famous people feel the same way or it becomes a trend then it is a problem for a social network's business.

Most of the time when you hear more than one comic tell the same joke its a case of convergent evolution. Only the scummiest of comics actively steal other's jokes and they tend to get ostracized for it.