That's a fairly common question, actually, and probably deserves a post of its own. There's no true consensus, because there's always that miniscule chance that some jagoff will steal your work, but my personal philosophy is not to worry and not to care.

As far as I'm concerned, the moment I share my world, my world is no longer mine--it's everybody's. I encourage people to play with it, take it, make it their own, make spin-offs and alterations, whatever. It would be high praise to me to see someone take something I've come up with and love it so much they have to do something with it themselves. Imitation is the most sincere form of flattery, as they say.

I've never had a bad experience with being stolen from. People have before asked me if they could use something cool I came up with, and I said yes. Others have seen that I've posted similar ideas to theirs, and they've been disappointed that their idea isn't original, and want to change it. There's an instance where someone wanted to be original so badly they actively pushed away from an idea because they saw it elsewhere. Ultimately, I think that's the way a lot of people operate. Most people want to create wholly original stuff. Most people don't want to steal.

When I was in high school, I wrote a short story and showed it to my best friend. He loved it so much he wrote a sequel. Then he showed both stories to a friend of his own, and she wrote a third piece to it. That was pretty awesome. I did not think one quick story could get so much attention, and breed so much more creativity. They took the original story and transformed it into something bigger and better.

I always wonder why people worry so much about their stuff getting stolen. Don't people realize who awesome it feels?!