Advertisers want to know how many people they can reach before they commit to a multi-million campaign. But in the mobile industry, determining the size of the audience is still difficult, if not impossible—especially among the dozen-or-so mobile ad networks.

In an effort to resolve this, Nielsen has reluctantly released a list of the six largest mobile ad networks in the U.S. However, in a strange twist, the research firm warns Mobile Marketer, which first reported the results, that it cannot stand by the report’s accuracy—because it’s so hard to tally a network’s true reach.

With that caveat in mind, Nielsen says the top U.S. mobile ad networks in terms of monthly unique visitors are:

1. Millennial Media: 45.6 million
2. AOL/Platform-A’s Third Screen Media: 28.6 million
3. AdMob: 25.7 million
4. Microsoft’s MSN Ad Network: 25.4 million
5. Jumptap: 23.4 million
6. Quattro Wireless: 23 million

Nielsen said there were 55 million unique visitors to the mobile web in June, meaning that at the high-end Millennial reached up to 82 percent of the mobile users, while at the low-end, Quattro Wireless is covering 42 percent.

David Gill, director of mobile media and advertising at Nielsen, explained that it’s difficult to compare the various networks because they don’t always “have 100 percent of their ads on each site.” That means, if a publisher uses multiple ad networks, then those ad networks would get credit for all of the visitors—not just the fraction of visitors that saw their ads. Another short-coming of the report is that Nielsen gathered the information from the ad networks, and cannot attest to the accuracy of the information. It also does not include the impact of mobile applications, which is a larger market at 62 million monthly visitors than the mobile web.

In the article, most of the ad networks say the report can be be used as a valuable guide and is accurate enough. The biggest protests came from AdMob and Quattro. AdMob said its figures are hurt by not including applications, while Quattro said the report is incomplete because it only takes into account select sites from its network.

Whether the list is deemed accurate or not, one thing to remember is that the report is measuring visitors, not ads served. Until the ad networks report their revenues publicly, we won’t know how successful they are.