If you're at critical mass, you might be able to hold your own. Foursquare and Twitter have enough users and industry clout that they can hold their own. If you're just starting on an idea, maybe you're getting a little bit of momentum, it might be tempting to talk to someone at Facebook and see if they will help you.
But… It's easier and less risky for Facebook to just implement your idea themselves. If they want a feature for their users, why depend on some little startup that may or may not fail? I don't think they're going out of their way to steal ideas. I think they are trying to maximize their own success. We could call it a lack of morals, but I think it's subjective which is why we have these debates.
Still, you probably should try to think about it from Facebook's perspective and be realistic about your position of leverage before you approach them with an idea.