Harvard study points out drive-by nature of Twitter usage. The median number of user twitter postings is one. 10% of users account for 90% of activity. So, for power users, it's perhaps more analagous to accumulating a large email list or generating a large number of RSS feed subscriptions....it follows then that those who do not generate large followings like celebrity-publicist driven efforts will have a dis-incentive to use the service over time. It depends on how the product evolves. However, It has surged in the consciousness as a new kind of sub-second communications tool. The idea of a short, attention-deficit safe broadcast to phones + computers was brilliant, though there may not be voluminous barriers to switching at some point in the future.
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