For many years, mobile has been the ‘next big thing‘ for advertisers. And to be sure, the market for mobile ads has grown by leaps and bounds in dollar-terms.

The latest figure evidencing the growth of mobile as an advertising medium: according to comScore, the number of advertisers in the U.S. running mobile campaigns has grown exponentially in the past two years.

When comScore looked at Ad Metrix Mobile data for 600 of the mobile internet’s properties in April, the number of advertisers was 689, an increase of more than 120% from two years ago.

Needless to say, if the market for mobile advertising to grow rapidly into the future, larger numbers of advertisers need to buy into the medium.

In theory, mobile will have a key role to play in most multichannel advertising strategies in the future, and the timing appears to be right now. Thanks in large part to the rise of smart phones and greater use of the mobile internet, advertisers are increasingly experimenting with mobile ads. And in many cases, they should be liking what they see.

According to a recent study, click through rates on mobile search ads are 2.7% higher on average than their desktop counterparts.

But there’s still a huge amount of room for growth. Right now, comScore says that the mobile content and publishing category accounts for 50% of mobile ads served, with consumer discretionary representing another 26%.

That means more than three-quarters of mobile ads cover just two categories. Lucrative categories, like financial services, aren’t as prominent — yet.

The key to continued growth of mobile advertising would appear to be continued smart phone ownership. According to comScore, smart phone users access their mobile browsers and mobile apps at much greater clips than their feature phone-owning counterparts, 82.3% and 85% to 19.1% and 15.9%, respectively.

Currently, 31% of mobile phone owners have a smart phone. But that number is increasing rapidly; last year, just 20% of mobile phone owners in the U.S. owned a smart phone.

The numbers make it clear: if the number of smart phone owners keeps going up, so too will the number of advertisers spending on mobile ads. In turn, publishers already active in mobile will see more opportunities to build ad revenue, and publishers not active in mobile will have greater incentives to develop a mobile strategy.