I recently came across a website belonging to a person named TA Lucas, who atests to being an avid worldbuilder of sorts. Now, he owns two forum based world building projects, associated with the RabiKi name, one being Fantasy oriented and the other [called Prime] being science fiction based.
Although I can't say anything about the fantasy side of this the SF project Prime has been inactive for quite a while, but it has had significant work put into it already so I would certainly call it established. It has only just reopened in the past week to "demand" but it still fairly inactive to the tune of the owner, one or two member [most inactive] and I being the only people there, but I would love to see this place more active once again as it could really be a fantastic opportunity/exercise in flexing your creative muscles.
Here's a link: Welcome to RabiKi
I appologise if shameless advertising like this frowned upon haha, think of this more as a testimony... or just some one seeing promise in an old project and wishing it could be active again.