Garry Tan has announced that he is leaving Posterous, the ultra-simplistic microblogging company he helped found in 2008. Tan wrote on his blog today that it was time to move on and that he would be taking an advisory role with the company in order to do what he was most passionate about - work with startups.

"My greatest passions lie with the early stage of building world-changing consumer products," writes Tan. "To that end, I've decided to join the team at Y Combinator as a designer-in-residence and help the dozens of top pre-seed startups in the newest Winter 2011 batch reach their potential through excellent user experience."

Tan has been involved with the startup community in San Francisco, most recently creating an interactive Google map of startup-centric neighborhoods in San Francisco.

Our own Audrey Watters calls the move "huge for YCombinator", relating how she watched Tan conduct back-to-back mentoring sessions for hours on end at a recent startup event.

Rafael Corrales, CEO of the Silicon Valley VC-backed startup LearnBoost, agrees that the move is a big one.

"It's amazing how Y Combinator has recently recruited some of the best guys in the Valley," said Corrales. "It's like a miniature version of what Facebook has done the past few years."

Posterous itself was a YCombinator-funded startup, so Tan will be returning to his roots in some ways. Tan writes that he feels comfortable that Posterous will be in good hands:

Just as Posterous has prospered, grown and changed, so to is it time for me to evolve my role. Effective today, I'm ending my day-to-day development with Posterous and moving into an advisory role. Though my day-to-day may change, my faith in the team and the product is unchanged and unwavering. Posterous is in good hands and on the right track to fulfilling its potential. I am proud of what we've built together and look forward to the future with anticipation to see where the team and you, the users, take this very special community. [...] I am greatly thankful to our team, investors and most of all our users for all the amazing work and adventures. Thanks for all of your support.

Good luck Garry!