Yep, that promotion is GDNet+ only -- the intent was to thank our paid subscribers and hopefully encourage some more subscriptions rather than everyone just gaining the benefits for free via Crossbones -- as a GDNet+ benefit it's also available to new members without having to gain 1,000 rep first.
Any member with an active GDNet+ subscription can place a leaderboard ad at no additional cost, which will remain in circulation for 50,000 impressions. Higher rep members (I'm afraid I'm not sure of the exact threshold) can place a second ad at the same time, while anyone else uploading a new ad will have it replace the first one.
Taking a random selection of the currently active ads we have the following figures:
31,462 impressions, 36 clicks
23,072 impressions, 38 clicks
21,726 impressions, 61 clicks
6567 impressions, 15 clicks
The highest number of clicks achieved was 1824, while the lowest was 5 (for a duplicated ad). I obviously can't tell you if those clicks resulted in sales, downloads, or whatever the members in question were hoping for, but at $5 for a month of membership (less for a crossbones member) it's pretty cheap to give it a go.
The ads with the best results are for developer resources rather than games, although a couple of games have made it into the triple digits of clicks.
We also have other ad units and the possibility of advertising in our email newsletter, which are not available through the above promotion -- if you'd like details on those shoot off an email to email@example.com and someone can help you out.
Hope that helps!