You own your original ideas, technically speaking. Just other people that view the board can use the ideas, copy it, or remix it into their own ideas as well.

As an analogy, say you buy a brand new red car, and you drive it on a public street. People who take a picture of the car don’t own the car, you do of course. They, however, own the picture of your car and can sell it, copy it, post it, whatever as they please.