Twenty-four hours after it introduced a new service that lets Gmail subscribers place phone calls from their desktops, Google reported that one million people had tried the service.

The service lets Gmail account holders to make voice calls to any traditional phone number from their Gmail account once they have installed of a voice and video plug-in. Unlike a similar service that Google offers for cell phones - Google Voice - users don't need a special number assigned to them to use it. For now the calls are free in the US and Canada and quite reasonably priced for international.

While Google has not revealed related ad vehicles, they surely are coming. A hint of what they might look like can be gleaned from some of the user data in Skype's recently-filed prospectus for its IPO, as well as its plans for monetization.

Skype could easily generate a new revenue stream of $12-$15 million annually if it were to introduce a homepage banner ad product with rates in the $8-$10 per thousand impressions, as YouTube is rumored to get, Fortune speculates.

However the real opportunity for advertising for Skype - and presumably Google given the similarity between the two services - will be found in ad formats that don't exist yet, Fortune goes on to say. Tom Bedecarre, CEO of the digital advertising agency AKQA, points to ad formats structured around lengthy video calls.

Targeting Facebook

These ad formats, though, are likely to be almost beside the point for Google. For while it would appear that Google has launched this service to make new inroads into yet another digital space - Skype's territory - the growing consensus is that Facebook and its social network is the real target, according to Goldman Sachs analyst James Mitchell.

"We assume Google's ulterior motive is less about disrupting the telecommunications and more about driving engagement within Gmail and its social-networking activities, to better compete with social networks such as Facebook," Mitchell wrote in a note to investors.