Shapeways is the most used 3D Printing Service in the recent Survey of the 3D Printing Community conducted with support by the P2P Foundation. 

Interestingly many of the 358 respondents had never used a service such as Shapeways to do their 3D Printing with many using a RepRap and it's derivatives like the MakerBot and Ultimaker. Also interesting was that for many 2011 was the first year they used 3D Printing for the first time and just as at Shapeways, the usage of 3D printing is broadly and evenly spread among many uses.

 Read the full results at peerproduction.net