The penetration of Internet usage in the US is about 80%, and the population is about 310m. Therefore, there are 248m Internet users. If you do not consider people about 90 or kids below 10, we may find about 200m Internet users. Since there are other solutions, e.g., Yahoo maps, if we assume Google has 60% market share for Google Maps, we find that there are about 120m Google Maps users.