Yahoo (NSDQ: YHOO) is closing down its one-year-old social networking site, Yahoo Mash. The site, which has been in private beta, will shut down Sept. 29. In a post on the site, via the Atlanta Business Chronicle, Matt Warburton, Yahoo Community Manager, writes that the company may have accidentally contacted non-Mash members about the site’s demise. No word was given as to why Yahoo was abandoning the project. As for the actual users — the number of those taking part in the test was not immediately clear — all of the content on their Mash profile will be unavailable after next month. But Warburton said those who have an account with Yahoo 360, the company’s current social net portal, will not be affected. Elsewhere, Yahoo says that users’ 360 profiles will be transitioned to a new system sometime in the second half of the year.