No Market Share movement for Posterous yet sorry.
Get a list of 6,803 websites using Posterous which includes location information, hosting data, contact details, 57,068 currently live websites
which includes 56,540 domains that redirect to sites in the list. 6,275 sites that used this technology previously and 371 websites in the United States currently using Posterous.
Distribution of locations of all Posterous customers we've ever tracked.