Just want to point out that your title says something different than your actual post. A meme would already be repeated everywhere. It will have mutated, and the Internet will claim it as its own. And SNL would not make something copying the meme but would instead comment on it.
As for posting something here like a funny line, joke, or idea for a sketch: unless it was really elaborate or specific, other people (including SNL’s writers) could easily have come up with the same idea independently. How would you know whether they got it from you?
Is a meme ever random text on a message board? I think of visuals when I think of memes, and this is the least visual place on the internet. Also, I don’t think of people creating something here, maybe creating something and then sharing it here.
The parody exemption is vastly overrated by most people. It really isn’t all that broad. For something like SNL to get away based on parody it has to be clear that they are making fun of something and no rational person would mistake the original for the parody.
If they were to use a joke that a Doper created and posted here, that’s not parody and it could be an issue.
The key points are theft of ideas and theft of the written word here. If they were to repeat something of non-trivial* length posted here without permission or credit, it’s a problem. If they decided to make a skit based on the idea of “What if Lord of The Rings were written by someone else?” then there could be a lawsuit but it wouldn’t be so easy to win.
What constitutes the minimal length of something to re-use before copyright law kicks in is not exactly specified. It would have to hashed out in court for each case.
Yes, a meme can be any information or idea that is repeated by different people. Many of the SDMB running gags count as memes; Og, 1920s death ray, Hey opal, just once for 20 minutes, and so on. Whether any of these memes are capable of spreading out of our message board culture is more debatable.
But somebody figured out that internet memes can become more virulent when you add visuals, and so those visual memes have become what a lot of people think of when they see or hear the word “meme.”