Yahoo!, Microsoft and AOL unveiled a joint advertising agreement designed to take on internet giant Google.

The deal will allow the ad networks operated by the three companies to offer some of their display advertising inventory to their respective customers.

“The partnership should enhance the demand for and value of each party’s display advertising offerings as well as provide better yield for both participating publishers and advertisers,” the companies said in a statement.

Google has been taking an increasing share of the lucrative and growing display advertising market, according to research firm eMarketer, and Yahoo!, Microsoft and AOL have all lost share.

Google makes most of its money from advertising tied to internet search but has been gaining a larger share of the revenue from online display advertising, which includes rich media, digital video and banner ads.

According to eMarketer, Yahoo!’s share of the US display ad market is expected to fall to 13.1 per cent this year from 14.4 per cent last year while Microsoft’s share will decline to 4.9 per cent from 5.1 per cent.

AOL’s share is expected to fall to 4.2 per cent this year from 4.8 per cent in 2010, it said. Google’s share of overall US display ad revenue will grow to 9.3 per cent this year from 8.6 per cent last year, eMarketer said.

Facebook, meanwhile, will see its share of US online display ad revenue grow to 17.7 per cent this year from 12.2 per cent last year, according to eMarketer.