Nielsen//Netratings announced their April 2007 U.S. search engine market share [PDF] ratings today. Some of the finer points of the report include:

• Of the top five, only Ask.com showed negative year-over-year growth (-2.3%)
• Google showed a 42% year-over-year growth in April

According to Nielsen, the search rankings go as follows:

1. Google (55.2%)
2. Yahoo (21.9%)
3. MSN (9.0%)
4. AOL (5.4%)
5. Ask.com (1.8%)
6. The remaining slots are split among My Web, Comcast, EarthLink, Dogpile.com, and My Way

It’s funny to see how so many different companies can have such different numbers for these rankings. Just a week ago or so I was reading another set of stats that showed Google’s market share upwards of 60%. It makes me wonder if we shouldn’t just add them all together and take an average or something.

Which ranking service or services do you use? What makes one service more trustworthy over another?