luka93, thanks seems an interesting idea!
So, for example, instead of having a simple "corporate" facebook page to where you post ads published in your website with a certain frequency (that's what I have today), you have a ads group to where you publish ads from your website directly. Is this it? And do you allow members of the group to publish in the group? If you do, what is incentive for members of the group to publish ads in both sides (facebook and osclass site)? What is the message you use to drive member of the group to your website?
By the way, when I have the time I run around the groups dedicated to the niche my site is dedicated to using some guerrilla marketing sending msg to group member like: "forguet the up's up's publish your ads at xxxx.com... " It doesn't produce any relevant results...