Good questions. My (partial) answers:

a) Blogs inside the firewall are examples, as well as enterprise wikis (Socialtext).

b) I've seen blogs used to provide feedback on courses from actual participants. Wikis work too. Each training coordinator can keep a blog to update on next courses and get requests for more training. Internal social networking software (Elgg) can be used to create affinity groups around issues or fields. Tag clouds can show emerging conversations around particular issues. This happens when blogs replace mass e-mail.

c) Facebook? I don't get it either. Linked-In works for many people for business networking.