Here's a great example of how Barrie North from Compass Desings / Simpweb is using Twitter as an effective marketing tool.  He's running "The great #joomla tweet-a-way". It's taken Barrie a couple of months on Twitter to get there.  What might look like a quick and easy web marketing trick is actually awell crafted strategy that leverages Barries reach as a blogger.

Update:

Google Uses Twitter to Sell Ads

15 Fascinating Ways to Track Twitter Trends