To be honest, I don't think that it's the duplicate pages which are the biggest problem, but at this point rather the on-page. Most of your potential users will search for "<> for sale in <<location>" so you should try to make sure that these kind of pages exist, with on-page elements. It will also help to make your site more flat - currently 46% of your pages are >=5 clicks from the home page. </location>
A part from that, personal ads sites tend to get a lot of traffic from long tail keywords, so the more ads you have, the more potential you have to attract traffic. Unfortunately, if you don't have a lot of traffic, you won't get a lot of ads, so it's a bit of a vicious circle you're in. When I crawled your site, I found about 450 HTML pages (unique), which remains quite small.
Not sure how the competition is in Spain, but normally it's a quite competitive market you're in, with a lot of players which are well established since quite a few years. So even if you do everything right, they will still beat you because they have a better link profile & reputation. It could be a better to focus on one specific niche (cars, motorcycles, boats,... ) and be really great on that niche, and expand to other niches once the first one gains momentum.
Social factors have some importance, but remember that these links are no follow, and that Google is not indexing all of Facebook.
rgds,
Dirk