Prior art, as defined by the USPTO, is claimed by previously published patent applications and letters patent, both here and abroad, as well as articles submitted in scholarly journals. Thus far it has not been established by any PTO Examiner I've seen that a blog/ social network/ forum/ static site is of the same measurable quality as a scholarly publication.
In the absence of this, all PriorArt (as you propose) would do is provide a submitter's unprotected intellectual property on a domain for everyone else to see and pillage.
It is reasonable to assume that trollers reading the forum/ blog could see a good idea and (if there is indeed no prior art existing for it after conducting a search) patent it for themselves... thereby restricting the orignal inventor from holding license to his/ her own ideas!
There is an idea sharing subreddit that's fairly popular: http://www.reddit.com/r/idea/
Actually it will. To avoid repetition:
... fairly similar.
Incidentally, prior art isn't something someone "claims." It's something a patent has to avoid.