Martin Kloos' "Comm.unities.of.prac.tice 2.0" examines how three different collaborative tools; blogs, wikis and social bookmarking, support learning in communities of practice.

http://www.martinkloos.nl/thesis-M.Kloos.pdf 

Solution Grove implements all three (plus forums) and I often struggle with how to explain to people which will work for their situation. Here is Kloos' take.
 

Social bookmarking focuses on informal information sharing and the creation of
a shared repertoire, blogs focus on meaning making, and wikis focus on knowledge
creation and collaboration. In other words: social bookmarking focuses on
informal information sharing, weblogs focus on informal socializing (e.g. interaction
and discussion), and wikis focus on formal knowledge creation and
collaboration.

He didn't study forums which I think support informal socializing, interaction and discussion better then weblogs.  I also believe Forums also support asking for advice and opinions while blogs support stating opinions.

So here is my bullet list view

• Social Bookmarking – Informal information sharing and collaborative resource library creation.
• Blogs – focus on meaning making, stating and discussing opinions
• Forums – informal discussion, Q&A
• Wiki – Collaborative formal knowledge creation