We're hearing some pretty amazing statistics about Twitter these days: growth from February 2008 to February 2009 was reportedly 1382%, with the incline increasing yet further in recent months.

But like many social networks, it seems many people lose steam with the service. Stat tracking firm Nielsen reports today that a full 60% of users who sign up fail to return the following month. And in the 12 months "pre-Oprah", retention rates were even lower: only 30% returned the next month. That's good news, to some degree: retention rates have increased over time.

But how does Twitter's retention rate compare to Facebook and MySpace in the early days? Not well, says Nielsen:

Compare it to the two heavily-touted behemoths of social networking when they were just starting out...we found that even when Facebook and MySpace were emerging networks like Twitter is now, their retention rates were twice as high. When they went through their explosive growth phases, that retention only went up, and both sit at nearly 70 percent today.

The question is, how can Twitter make sure that users stick around for the long haul? What is it about Facebook and MySpace that make them so appealing? Could it be, possibly, that finding friends on Twitter remains harder than doing so on other social networks?

UPDATE: As discussed in the comments, Nielsen is only able to measure return visits to Twitter.com: how many people set up a desktop application like TweetDeck and continue to Tweet, but never return to Twitter.com?