If you look at the URL for a Typepad comment (a comment on a Typepad blog), you can see that it has an id: for example, this URL from a comment on The Amateur Gormet
As far as I can tell, these ids are global, so if we sample them, we can plot the ids (let's assume that id N represents the Nth comment globally in the Typepad universe). Taking data from The Amateur Gormet blog, and Micropersuasion, we can get a graph reaching from 2004 to the present day. The graph looks like this:
Having the same data for the number of Typepad blogs over time would make this anlaysis really interesting...