Major Internet Backbone Companies:

(Companies are listed in approximate order of size.)

UUnet/WorldCom
AT&T
Sprint
Genuity
PSINet
Cable & Wireless
XO Communications
Verio
Qwest
Global Crossing

So how did UUnet go so huge, as the world's biggest backbone and wholesale
Internet provider by far? Simple: They were the first. In 1987, UUnet was
founded as the world's first commercial Internet access provider; Prior to
that, the Internet had been a non-commercial network used mainly by
universities and the government.