Keep in mind that most of these companies have to work just as hard once they finish YC. It generally takes a few more years after YC to really make something of themselves. Adding on to that, some companies take awhile to develop an efficient revenue stream. From what I hear, Reddit still has a hard time making money even though it raised $50 million in investments and was bought out by Conde Nast. Sometimes the investments made are seen as a promise for the long-term rather then revolutionizing the way we live tomorrow.