Nielsen Online reports that Google has seen its US search market share fall from 57.7 percent to 56.3 percent in December while Microsoft gained nearly 2 percent, rising to 13.8 percent from 12 percent.
Number two Yahoo was the other loser, dipping slightly from 17.9 percent to 17.7 percent.
Although Nielsen says that a new methodology it implemented might explain why Google portion of the market decreased, the results were corroborated by Alexa's ratings.
Today's results show that Youtube is currently the second most visited website on the Internet behind Yahoo with live.com in third and Google.com in fourth place.
Alexa calculates market share according to domain name and while Yahoo and Google put all their core services under one domain name, Microsoft on the other hand uses different domain names.
Microsoft uses MSN.com and Live.com for search for example.
Nielsen also noticed that Microsoft offered prizes such as Games and T-Shirts to get US surfers to use its services, which might explain partly the rise in December.