Patents are largely useless if you're small. They're very expensive to get, and once you have them even more expensive to defend. Moreover, a defensible patent is so narrow in scope that it provides very little protection by itself. Someone makes a small change to your idea and bypasses the patent entirely.

If your idea is good and you execute well, you have little to fear from large competitors. The larger the company, the harder it is for them to innovate. They're protecting their revenue streams and have less appetite for risk.

They'll wait for your idea to be proven before they consider implementing it, letting you absorb the risk. When it comes time for them to implement, you may have a large enough market share that it doesn't matter what they do. Or you may have made enough progress that it's easier for them to acquire you than it is to build it themselves.

One thing that does concern me in your description is it sounds like your idea is simply a feature, not a viable business or even a useful product. The fact that you're concerned with your idea being easily tacked onto an existing social site leads me to believe that it's really just an improvement on things that are already out there.

Features don't really stand alone by themselves. You need a compelling product and a decent revenue plan to turn it into a business.