What are all these other tablets being used for? Not for viewing ads, that's for sure.

Tablets grow, while the iPad grows even more. Tablets accounted for 28% of all mobile ad impressions over Q3 2013, which was double their share in Q4 2012. Within the growing tablet format, Apple’s iPad, already the dominant device, further established its position by gaining share.
Apple pulls away from the pack. Apple remained the strongest manufacturer by volume, with 63% of all global ad impressions. Furthermore, Apple also increased its share of global impressions against the ‘long tail’ of manufacturers
- that is, all mobile device manufacturers other than Apple and Samsung.

From the Q3 2013 Adfonic Global AdMetrics Report. Full PDF report linked here: