When setting up a Social Marketing Channel rule, the system provides 50 domains by default. That list does not contain metacafe.com, pinterest.com, t.co, google+, ilike and hulu. Those additional domains are listed in the Social Analytics - Social Platforms report. But at the same time, metacafe.com is NOT counted as a social referrer in the referring domains report.
By default, social networks should be in sync across all 3 reports. I like the flexibility of the Marketing Channels report to add or remove domains, but default set up should mirror social in referrer type & social platforms
And just in general the Marketing Channels list should be updated regularly - the fact that pinterest and t.co are not in there is not good.