Of the 97 million U.S. adults who own a smartphone running Apple iOS, Android or BlackBerry operating systems, 45.4%, or 44.0 million, visited Amazon.com Inc.’s m-commerce site or used the e-retailer’s apps in March, according to the new Mobile Metrix 2.0 report from web and mobile measurement firm comScore Inc. 28.0%, or 27.2 million, visited eBay Inc.’s m-commerce site or used the marketplace’s apps.

85.7% of the time these smartphone users spent with Amazon was with apps, while only 14.3% was spent on the mobile site, comScore says. With eBay, apps accounted for 82.4% of the time spent and the mobile site 17.6%. The results can offer guidance to retailers developing mobile commerce programs: For the top 15 Internet destinations measured by comScore, 81.5% of time spent was on apps while 18.5% was on sites.

Following are the top 15 mobile destinations, the number of unique visitors in millions and the percent reach, according to comScore:

• Google Inc., 94.0, 96.9%
• Facebook, 78.0, 80.4%
• Yahoo sites, 66.2, 68.2%
• Amazon.com, 44.0, 45.4%
• Wikimedia Foundation sites, 39.1, 40.3%
• Apple Inc., 38.3, 39.5% (99.7% of visits were through its iTunes and App Store apps)
• Cooliris Inc., 28.5, 29.4%
• AOL Inc., 28.0, 28.9%
• eBay Inc., 27.2, 28.0%
• Zynga, 26.6, 27.4%
• Twitter, 25.6, 26.4%
• Rovio (Angry Birds), 25.1, 25.8%
• The Weather Channel, 24.1, 24.9%
• Microsoft sites, 24.0, 24.7%
• ESPN, 23.3, 24.0%

On the social media front, smartphones are driving significant consumer engagement. 80.4% of iPhone, Android and BlackBerry users in March accessed Facebook through its mobile site or app, 26.4% Twitter, 7.9% LinkedIn, 7.7% Pinterest, 5.7% foursquare and 4.6% Tumblr, comScore finds. Smartphone visitors to Facebook spent on average 441.3 minutes on the social network’s site or app in March; 114.4 minutes on Twitter, 12.9 minutes on LinkedIn, 52.9 minutes on Pinterest, 145.6 minutes on foursquare and 68.4 minutes on Tumblr, comScore says.