I wouldn't burn too many candles hoping to get in or trying to figure out a reason why you did not. There is a benefit to applying in that it makes your commitment to your startup even stronger.
Focus on what is important and that is your business. I truly believe building something sustainable is better for YOU, than trying to make the next Dropbox, Facebook etc. But if you already gaining Facebook like traction I am sure YC will already have heard of you.
Worry about your startup and just focus on making it sustainable. Becoming popular with customers and maintaining that relationship. has more benefits for more people than getting into YC. Focus on what is important.