With Bing now powering Yahoo, Bing’s combined powered search accounted for 24.56 percent of U.S. search traffic for the week ending August 28.

Experian Hitwise says that that Google accounted for 71.59 percent of all U.S. searches during the period, with Yahoo! Search and Bing receiving 14.32 percent and 10.24 percent, respectively.

Year over year, Bing has seen double-digit growth in Automotive, Health, Shopping and Travel category searches, with shopping experiencing a whopping 66 percent increase.

I’ve heard from various sources that Bing traffic also tends to convert a little better.

Bing is now too big to ignore and can also be a lifeline to merchants not doing too well on Google – so it’s certainly time to optimize for Bing.

It’s good in the sense that either Google or Bing can help sustain an online merchant – before Google was really the only show in town. However, commiserations to those merchants who were ranking well on Yahoo prior to Bing powering Y!’s results, particularly if they weren’t ranking well on Bing, as they would be feeling the pinch now.

As an aside, Experian Hitwise also found searches averaging five to eight words long decreased 2 percent from July 2010 to August 2010. Four- to seven-word searches decreased 2 percent.

Two-word searches are the favorite, amounting to 23.71 percent of all queries.

Searchers were becoming more savvier in that they were using more keywords to find the information they wanted, but these results seem to indicate search engines are getting better at returning desired results on fewer search criteria.

The data from HitWise was based on a sample of 10 million U.S. Internet users.

Read more of Experian Hitwise’s report.