There was a blog post here by that title or similar. May replace "Facebook" with the "next big thing" or "accelerator" with "Y Combinator". But my Google search skills and HNsearch isn't helping me out here. Anyone bookmark it?
<a href="http://www.quora.com/Technology-Trends/What-will-be-the-next-big-thing-after-Facebook-and-Twitter-1" rel="nofollow">http://www.quora.com/Technology-Trends/What-will-be-the-next...</a><p>Is it this? One of the answers matches your phrasing.