MikeDXUNL

I had a question and couldn't really pin-point a forum to put this post in, so I hope this suits my topic. You always hear "Facebook is rolling out a new feature"; most recently "Twitter is rolling out the New Twitter.com" but it only releases to so many people at time. Or friends will get features on Facebook before I do (like I couldn't like threaded comments until about a month after all my other friends could). I was just wondering if anyone knows the actual method or how large sites go about releasing new features to users?