Forums should be like social mixers. Everyone is at equal level, milling about and discussing various topics with others. Forums 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 the comments area allows questions and comments from the audience. Kind of like a Q&A session after said keynote.
Blogs are journals often authored by one individual, and sometimes teams. In the context of business communication, these are often used to highlight exclusive content and talk with the marketplace, joining the conversations that existing external bloggers are having.
Social Networks are like topic tables at a school lunch. Imagine a fresher’s fair with big white signs above big long tables inviting people to sit down and join up with others of a like-minded interest? It’s like that.
Social networks allow people to focus on a person’s relationships or interests, rather that just their focused on topic. People that know each other (or want to meet each other) can connect via a variety of common interests. These are great tools to get people of like interest to connect to each other and share information.
In this way, it’s possible to concentrate on the selling points of each and hopefully bridge people’s expectations when using them.