Facebook would be an obvious choice and has a substantial market share within the social networking industry.
In a recent survey completed by over 1,000 marketers, it was noted that 75.3% of B2C businesses use Facebook more than any other social networking site in order to generate sales, leads and website traffic.
However Google+ received a massive level of growth when it first launched and has an active user count of 170 million.
In regards to what site "brings in the most work" in entirely depends upon the product or service that you are advertising. For instance a vast majority of Google+'s users have roles within the technological industry and is popular among technology experts. However sites such as Facebook have a vast audience, far greater than Google+, thus is essential that customers are targeted accordingly. Failing to do so may spread an advertising campaign very thin which is a marketers worse nightmare.
Looking at social media sites in a broad view, I would say ads need to be specific so as to relate to the audience and encourage the viewer to click on it. Facebook does a great job of doing so by allowing advertisers to heavily customise every aspect of the ad and target audiences effectively.
There are always other opinions including the use of Twitter and the upcoming Pinterest site although it is fair to say Facebook has a more established audience and is more likely to provide you with results. I wrote a blog post for our website investigating the effectiveness of Google+ if you want to look at that for further information.
Hope my opinions and collection of research has informed you further as to the more effective social media website of which to advertise on.