I like how you can brand the Share links to a version of your domain (i.e. contest.domain.com). However, since we can host all pages on our site, wouldn't it be possible to actually have the share link generated be ON our site and have your scripts pick it up for tracking (like they already do)?
With the point of this being viral sharing, it should generate a lot of social media links. These should support our websites profile in Google, etc. But having them all point to a subdomain that doesn't have other content, is at the best, not as valuable.
Plus, if we use our actual site domain (and landing page) for the share links, we don't even have to setup a CNAME. But most importantly, all the links people share on Facebook, Twitter, etc will point to our main website and domain, helping to build our Authority with Google.
Also very interested in this!