Websites have been slow to make their content available in mobile-optimised formats, according to market resaerch firm eMarketer estimates.

Emarketer said that more than two in five mobile users will go online from their phones each month, but most websites are not keen to leverage this fact to their advantage.

The company said research from mobile site detection API provider Company Data Trees found that in January 2011, 14.32% of the top 10,000 sites on Alexa had a mobile version, and 14.67% of publishers had one. Emarketer added the percentages have grown since then, to around 24% and 26%, respectively, as of September. That growth represents a 75% increase in mobile adoption among publishers and 66% among all websites over the course of approximately nine months.

Yet while more publishers are optimising their sites for mobile, most are still not serving mobile ads, said eMarketer. In September 2011, more than 61% of the publisher sites studied by Company Data Trees had no mobile ads, said eMarketer.

Company Data Trees CEO David Engel told eMarketer that publishers should pay more serious attention to this, and to creating a mobile website.

"So much weight is put on apps, but the real opportunity is for the mobile website," said Engel.