ComScore: iOS users on AT&T use WiFi a lot.

I removed the UK data from the charts (via comScore) for this post, but you can read the whole article and find the complete data here.

A look at both of comScore's charts, and you can quickly extrapolate that most iOS users are on AT&T's network.  The second chart shows that AT&T has a significantly higher percentage of its users who frequently use both mobile networks and WiFi.  This seems to reinforce the notion that AT&T's network is the slowest, most congested network on its backbone (the fiber part).

It also shows that speed does make a difference, since most iOS devices are on 3G networks (AT&T's iPhone 4S runs 14.4Mbps HSPA+, but much slower on other networks), while the opposite is true of Android devices, which run at 4G speeds on all of the networks.

(For fun, ask your iPhone friend to do a search....then pull out your 4G Android and tell them that they're taking too long.  Keep doing this, and then watch how their behavior changes.)

Mobile and Wi-Fi Internet Connection Activity Across iOS and Android Smartphone Platforms February 2012 United States Source: comScore Device Essentials
Smartphone Platform	% of Smartphones that Browse Only via Mobile Networks	% of Smartphones that Browse via Both Mobile and Wi-Fi Networks
iOS	29%	71%
Android	68%	32%

Mobile and Wi-Fi Internet Connection Activity Across Carriers February 2012 United States Source: comScore Device Essentials
Carrier	% of Smartphones that Browse Only via Mobile Networks	% of Smartphones that Browse via Both Mobile and Wi-Fi Networks
AT&T	42%	58%
Verizon	68%	32%
T-Mobile (U.S.)	68%	32%
Sprint	71%	29%
Total U.S.	62%	38%