Look at it this way: my neibour is a member of my social network since fromt time to time I will talk to him about specific issues but other than being neibours we do not have much in common. The social aspect of the relationship is quite involuntary.

On the other hand you and I, even though we never met, are both members of the learning community because we both share a common intertest and we give and take in order to foster the advancement of this community.

This is the same for social networks: they are social simply because all members share the platform, the way they use it is irrelevant.

Social is about being there, community is about contributing.