Brian Solis has a very helpfup post on hie PR 2.0 blog at:
http://www.briansolis.com/2009/10/the-top-100-twitter-publishing-tools-a...
Here's the post:
The Top 100 Twitter Publishing Tools and Services
By Brian Solis
* October 14, 2009
As I was writing the report on Facebook and Twitter traffic growth, I had noticed that the engagement time at Twitter.com had dropped by 31 percent year-over-year.
September 2009: 18:07
September 2008: 26:12
Engagement Difference = -31%









