**Deprecated and will be removed after September 30th, 2016**
Social Activities represents a social action (e.g. +1, comment, vote, share, etc.) that references content on your website but does not originate from your website. For example, if somebody post a comment on your content that has been shared by somebody on Google +, you will receive a notification in Google Analytics.
Social Interactions represent user interactions with social buttons and widgets on your site, for example, +1 button or Like on Facebook.
So, Social Activities occur offsite, while social interactions happen on your site.