As Larry Page goes about reorganizing Google to his liking, he might do well to transfer a few extra engineers to the search team. Google’s search engine of course continues to be dominant in the U.S., but new data from Experian Hitwise suggests it’s lost ground to the Bing-Yahoo alliance six months in a row.

Trace the decline for yourself if you like. Google had a market share of 72.15 percent in September, according to Experian Hitwise. Then that fell to 70.68 percent in October, and it’s been dropping ever since. The latest report pegs Google’s market share at 64.42 percent.

Meanwhile, Bing and Yahoo have been posting gains on a regular basis, and Experian Hitwise believes their combined market share exceeded 30.00 percent in March (making for a month-over-month gain of 5.37 percent).

That’s an impressive achievement. What’s more, Experian Hitwise stated, “Yahoo! Search and Bing achieved the highest success rates in March 2011. This means that for both search engines, more than 80 percent of searches executed resulted in a visit to a Website. Google achieved a success rate of 66 percent.”

All of this data seems bound to attract the attention (and money) of at least a few advertisers. It’s possible there will be a small snowball effect among users, too, as more people become curious about Bing.

Again, we’re not going to pretend this pattern marks the end of Google’s dominance. Fans are sure to find the trend worrying, though, and it’ll be very interesting to see how long the slide continues.