Even in the pre-Internet days, humorous responses to events showed a degree of uniformity. The "Who let the woman drive?" gag about the Challenger disaster was universal and instantaneous. How did these lines make it around the world? Answer: It was a coincidence. Different people thought the same way about the same events, independently of each other, and shared the result with their friends.
An example of this occurred today on Facebook. Two friends, one a professional comedy writer, one an old schoolfriend with no professional connection to the gag trade, each made the observation that, now that Kraft is taking over Cadbury's, they didn't fancy the idea of Cream Cheese Eggs. It's a great line. And it occurred to two un-connected (unless you count me as a common denominator, which would only work if they knew of each other's existence) people at roughly the same time. Which is why, unless the plagiarism is verbatim or the concept is so out there as to be instantly identifiable, it's problematic when comedians accuse each other of stealing material. A lot of comedy is simply a clever reaction to something, and, being human, we shouldn't be surprised if a lot of people react the same way, completely independently of each other.