The author suggests that it is not a factor in this arena as Tumblr will inevitably plateau due to a lack of new user signups. I disagree. So long as there are new users signing up for these services they will most likely lean toward Tumblr as the larger community is enough reason for them to go that route. I believe it will be Posterous who plateaus as a result of this.
As it stands, Posterous should be content in stealing Tumblr's users. And boy have they capitalized on this. Posterous allows you to import your old content from over 15 services (including Tumblr).
Facebook has shown us that there are over 500 million potential "new users." I'm not sure it's safe to say that Tumblr new user signups will plateau as a result of a lack of people showing interest in microblogging.