Mobile advertising remains small, but the bet is that it's going to explode over the next several years as more people buy smartphones and use the Web on the go. Search advertising is particularly expected to take off.

While search advertising was just 24% of the $160 million U.S. mobile advertising market last year, Kelsey Group projects it to grow to 73% of a $3.1 billion mobile ad market in 2013.

That's good news for Google, assuming it can capture the lion's share of the mobile search market. (It's well on its way.)