Try doing a trace route to several different sites on the internet, you'll find that -- so long as your traffic is hitting the US at some point -- your information is crossing AT&T's networks.
This applies to a large majority of traffic in and to, and even through, the United States. Because of the way traffic 'finds the quickest (or best) route possible' you're almost guaranteed to pass through one of their points of presence.
Since this article was published it's likely that other providers were pushed into doing the same thing. If I'm not mistaken, there was even an exec. shake-up at Verizon as a result of them not wanting to concede to the NSA's demands.
