Not strictly "social" per-se, but there were a number of cross overs from iGoogle. Also relevant as Now is undergoing some historical changes of it's own..
- Pixie Labs ended up moving their games to Google+ Games (remember Farmville?)
- The gadget infrastructure from iGoogle (GGS) was used heavily for the +1 button, OpenSocial, PeopleSense and others. +103390686048441232976 might even still carry a pager for this old infrastructure...