the main page (http://proofs.wiki/index.php?title=Main_Page&action=history) is from December 5, 2014 (first revision is timestamped in 2011, but that is the MediaWiki default page)

http://proofs.wiki/index.php?title=Prerequisite:Copyrights&a... shows the copyright page is from December 6, 2014.

So, I assume this started around two months ago. The sparseness of content is not in conflict with that.

I think I would rather use a site that uses a formal proof checker behind it to verify committed theorems. It may get populated slower, but it can better be trusted.