I've read a lot about Social Media. There are basic features mentioned in each and every article about SM's fundamentals: honest, short, informative, and fun.
One, however, is so obvious we fail to mention it: consistency.
You could have the greatest, most honest, most funny posts, but if you only do them once a year you're virtually invisible. Now, do not sacrifice quality for quantity, but if you have one post on your blog then you are virtually invisible.
The other downfall of inconsistency is that you're unable to work out your SM muscle. Web 2.0 is an ever-evolving beast that takes quite a bit of effort, patience, and sharpness to keep up with. If you're not on top of your game, you simply won't be able to keep up with the pace. Many people don't know that MySpace is all but cooked for any one other than musicians. Anyone staying on top of trends knows this.
Think about it like this. In the real world, you wouldn't disappear for a week without telling any one. You wouldn't stop communicating with your friends and family. Hopefully anyway. You can't build worthwhile relationships without consistency both online and off.
A great example of consistency that I've seen is she-conomy.com. This writer posts often on her blog and Tweets about her great posts. They are fun, informative, and consistent. Keeps me coming back.