Verizon released the first subscriber figures for its rapidly expanding 4G network today – presumably to highlight strong sales of the carrier’s first 4G handset, the HTC Thunderbolt.

Of the 565,000 (500,000 is the number Verizon added to its subscriber base in Q1) 4G users, about 260,000 are utilizing Verizon’s LTE service via an HTC Thunderbolt. This means that since the launch of the Thunderbolt on March 17, Verizon has been gaining over 100,000 new LTE subscribers a week. And with 4G devices like the Samsung DROID Charge and the Motorola DROID BIONIC on the way, it seems Big Red’s 4G campaign is just getting started.

These figures lie in contrast to Verizon’s primary 4G competitor, Sprint, which by the end of 2010 counted among its subscribers over 3.3 million 4G phones or mobile broadband devices. Much debate has occurred regarding the robustness of Verizon’s 4G LTE network, and just how much its relatively low number of subscribers is impacting speed at the moment. The argument from the Sprint camp has generally been that Sprint’s 4G network is hugely more burdened than Verizon’s, skewing results of speed tests comparing the two.

Verizon’s 4G subscriber base currently sits at around 20%* of Sprint’s, which is a sizable fraction, though it sounds much more dramatic in comparison if you were to say Sprint has five times the number of 4G subscribers as Verizon. Either way, there’s clearly some ground Verizon has to make up before it can win the "speed king" crown in the eyes of more skeptical consumers.

Still, Verizon’s plan to move to tiered data options could mitigate the influx of traffic from new 4G phones as customers monitor and self-restrict their usage to avoid overages. With Sprint’s Clearwire 4G expansion stalled until the two companies come to a new agreement (along with possibilities that Sprint may introduce LTE 4G), T-Mobile’s potential acquisition by AT&T (and thus the end of its HSPA+ 4G network), and AT&T’s LTE network still on the horizon, it’s clear no carrier has yet established dominance in the 4G market.

Verizon’s slew of 4G-related advertisements have certainly won the carrier a sizable chunk of early adopters, and if it can keep its network within advertised speeds (5-12Mbps) as the number of 4G subscribers grows, the nation’s largest carrier could soon be its largest 4G carrier as well.

*Number based on ~565,000 VZW 4G LTE subscribers at the end of Q1 2011 versus 2.4 million covered Sprint 4G subscribers at the end of 2010, with some leeway in Sprint’s favor for the 3 months separating the two.