I mean, GameFAQs had it too, but I only associate it with Reddit now, oops.

You get it by plurking a lot!

Which can be defined as either making plurks or responding to other people's plurks.

Although I think you lose karma if you don't do a top-level plurk at least once a day.

Which is why sometimes you might see people being like "KARMA PLURK" on their timelines.