Fads come and go when the proverbial flock loses interesting, and all the sheep drift to another, greener pasture.  This appears to be the case with micro-blogging service Twitter.

With the initial celebrity exposure, it's no wonder Twitter became such a sensation, but the majority of users don't actually stick around.  Just a few weeks ago, Twitter reportedly had a 131 percent increase in U.S. visitors alone, between February and March 2009.  Earlier this month, traffic was up by 700 percent, year-over-year; this boom has been attributed to an influx of middle-aged users in addition to media attention.

However, a report from media tracker Nielsen Co. revealed that 60 percent of Twitter users actually don't revisit the month after they join, and even at the height of its popularity, the site hada meager retention rate below 30 percent.  By contrast, Facebook and MySpace both have a retention rate above 70 percent following their popularity growth.

"Twitter just doesn't seem to have a whole lot of stickiness to it," said Ezra Gottheil, an analyst at Technology Business Research Inc.  "People join Twitter ... and then it has to be an on-going commitment. Twitter's charm is that it's immediate and instant. The minute you get busy, it's usefulness fades." 

He goes on to add, "It's a major head scratcher because it's intrinsically a slice of time.  I hope they reach a phase of general stability. It has to level off at some point and it won't level off at everyone in the whole world using Twitter."