No Market Share movement for Posterous yet sorry.
Get a list of 6,765 websites using Posterous which includes location information, hosting data, contact details, 499 currently live websites
and an additional 128,558 domains that redirect to sites in this list. 6,266 sites that used this technology previously and 344 websites in the United States currently using Posterous.
Distribution of locations of all Posterous customers we've ever tracked.