This is a Google offering that only allows Facebook and Twitter sign-in. That's a bit odd; perhaps it exists for the purpose of getting people to opt their Facebook/Twitter info over to Google? More evidence for this theory: here's what Prizes asks for via Facebook Connect: …name, profile picture, gender, networks, user ID, list of friends, and any other information I've shared with everyone… Prizes may access my dat…
What would Google gain from such practices? Why not just ask you your facebook/twitter username upfront? Propally only a small percentage of their user base is ever going to use prices.org anyway. I think the story here is that it is done by slide, which still acts like it's an independent company.
The idea this is Slide showing its independence is interesting too, but why no Google/Google+ as even an option?
I think one of the potential problems with Google+ is that it is a "brand extension", too close to the parent brand, which already means something else to people. (To some extent, this was the problem with "Google Video" versus "YouTube", as well.) Maybe Google has empowered Slide to move full-speed ahead on a "Plan B", a network of many social sites unencumbered by Google branding. At the extreme, if all else fails, this could culminate in a new social network service by Google but under a non-Google name.