A nice, common sense
post from Meryl Evans on WebWorkerDaily identifies the rules of thumb for social media. In essence, there really aren't any.
Having said that, Evans approaches social media smartly and has some discipline about even the free forms of it. Among her rules:
1. Do it the same time daily.
2. Spread the Tweets.
3. Join the right Twitter chats.
4. Review Facebook daily.
5. Update Linkedin at least a few times weekly.
6. Write at least two blog entries weekly.
7. Read others' blogs.