Forums are like social mixers, where everyone is at equal level, milling about and discussing with others. These many to many communication tools allow anyone to start a topic and anyone to respond to one. Members are often at equal level, and content is usually segmented by topic (rather than by people).
Blogs are like a keynote speech where the speaker (blogger) is in control of the discussion, but allows questions and comments from the audience. Blogs are journals often authored by one individual, and sometimes teams. In the context of business communication, these are often used to talk with the marketplace and to join the conversation that existing external bloggers may be having.
Social Networks allow members to organize around a personal relationships or interests, rather that just focused on topic. People that know each other (or want to meet each other) will connect by a variety of common interests. These are great tools to get people of like interest to connect to each other and share information.
