After trying searching and digging into these concepts, I found some points help me to understand them more clearly.
A social network is a social structure made of individuals (or organizations) called "nodes," which are tied (connected) by one or more specific types of interdependency, such as friendship, kinship, financial exchange, dislike, sexual relationships, or relationships of beliefs, knowledge or prestige.(From http://en.wikipedia.org/wiki/Social_network)
Social network analysis views social relationships in terms of network theory about nodes and ties. Nodes are the individual actors within the networks, and ties are the relationships between the actors. The resulting graph-based structures are often very complex. There can be many kinds of ties between the nodes. Research in a number of academic fields has shown that social networks operate on many levels, from families up to the level of nations, and play a critical role in determining the way problems are solved, organizations are run, and the degree to which individuals succeed in achieving their goals.
A virtual community, e-community or online community is a group of people that primarily interact via communication media such as newsletters, telephone, email, internet social network service or instant messages rather than face to face, for social, professional, educational or other purposes.
These definitions may make us more confuse. However, in my opinion, these are some things that we should consider:
1. Target of a social network is "building the social structure". Target of an online community site is: "building communication tools of a community". Anyways, in many cases the boundary is not always clear.
2. All activities of a user in a social network must be attached with "social nodes". That means "a user joined to a social network must find and expand his/her relationships". Without doing this, he/she can not see any activities are happening. This is the strongest point of social network ("encouraging and forcing making relationships"). In online community site, this is not required. You can join a community site (like Community Server forum) and can see all happening activities without making friends with anyone. (I found this point in this article).
I am waiting for your comments and sharings about these concepts!