Audience measurement firm comScore has released its May 2010 U.S. Search Data report, and it shows continued market share gains for Yahoo and Microsoft.

Yahoo and Bing/MSN each added approximately 60 bps and 30 bps to 18.3% and 12.1%, respectively. Google is down, claims comScore, declining approximately 70 bps for the second consecutive month to 63.7%.

But that’s not the whole story, and investors need to caution when interpreting the data as presented by comScore, say analysts.

Gleacher & Company’s Broadpoint.AmTech puts it this way:

While these numbers are correct on an apples-to-apples basis (in the sense that certain types of searches – e.g. contextual shortcuts and slide-shows – are being counted consistently across properties), the trending data for which we think comScore is most useful shows a different picture.

J.P. Morgan has this to say about the reported numbers:

User interface changes continue to cloud the picture. Google, Yahoo! and MSN all made notable changes in April and May, according to comScore. As such, numbers may not be directly comparable to past months.

We think more months of data under the new methodology could clarify matters.

We’ve detailed how Yahoo has boosted its search market share with these ‘tricks’ last month.

When adjusted, backing out Yahoo and Bing/MSN’s use of contextual shortcuts and image slide-shows from both May and April, Broadpoint.AmTech estimates that Yahoo’s share actually declined roughly 30 bps month over month in May to 16.6%, while Microsoft Sites’ share was flat at approximately 10.8%.

Google, after a small data collection adjustment to the April data (namely a change in how Google handles searches with typos), appears to have gained roughly 30 bps of share in May to 66.4%, says Broadpoint.AmTech. However, Google’s domestic core search market share was 63.7% in May, down slightly from 64.4% in April, J.P. Morgan claims.

According to the reported data, total US core search volume increased 11.2% year over year in May, an acceleration from 5.3% growth in April, adds J.P. Morgan. However, adjusting for the impact of user interface changes, the firm estimates search volume was up roughly 7%.