Also you may consider that only a small fraction of the worlds population (<20%) is using the internet, especially in developing countries. Typically, in countries with weak economies, the available hardware resources for users is limited and I can not think if a service less demanding on hardware than Twitter is.

Given that influential people, especially among those not already using the internet, recommends that in their communities, the current growth may seem small. New services may emerge, too so microblogging/social media will be more fragmented. Twitter may not be the correct answer for everyone.

It may be that Twitter's appeal is less board than Facebook or MySpace. That doesn't mean Twitter won't be hugely successful but perhaps not the 250 million users that Facebook has attracted.

It may be more accurate to look at those numbers and suggest that Twitter's adoption cycle is often quite a bit longer than with other SN. Despite all the help the 140-character constraint and simplicity of the service brings, members have to get over thinking of Twitter as just a broadcast of the mundane. Having an account already created makes it that much easier to join in earnest later.

I've noticed many of my old high school friends (from the 1980s) are going to Facebook when they start diving into social networks. Twitter is still a big conceptual leap.