I've seen plenty of press come out around all the startups that came out of ycombinator this year. I haven’t seen much about the twitter startups. I’m interested because our startup http://keytweet.com applied under that RFS. We have, like the other startups, been working hard all summer. I'd really like to see how we stack up against the companies that did get YC funding. We are working on a major redesign...so stayed tuned.
I think people are catching on that building a company that's totally dependent on an third party whose interests aren't intrinsically aligned with yours is dangerous. Sometimes it works out, but it adds a lot of risk.
So, "things built on Twitter" isn't more popular because you're throwing yourself at the whims of another company which isn't a very solid foundation for a business.
Well, my 'Built on Twitter' micro startup (a twitter client for feature-phones) hasn't been quite a success I was hoping for. It has attracted some users, and still hovers somewhere at the bottom of top 100 clients, but after I realized I don't have a good way to monetize it, I decided to leave it and move to a greener pastures.
So, "things built on Twitter" isn't more popular because you're throwing yourself at the whims of another company which isn't a very solid foundation for a business.