Come back to normal Price Levels. In some niches it is that something that doesn't cost anything, is not worth anything.
Did you try Advertising on other 'social' sites? Linkedin Comes to mind.
My niche has pretty much two types of players: the big online retailers who charge full tilt, and the smaller retailers who compete on price. If I raise prices by $10 or $20 on some $200 to $400 items, I lose out to other smaller retailers.
For several years I had to attend traveling public "shows" in my niche, where independent dealers sell to the public. One dealer liked to goof on people He put a length of clothesline across his table, and had clipped to it several brand new crisp $1 bills. He put a sticker on each one for 95 cents. He had several people ask what was wrong with them. Some people offered him 75 or 80 cents. They were serious. That's the mindset. I've lost out on sales because of $5 on a $200 to $500 item, even though I have five-star ratings and the other retailer didn't.
As for the types of ads, I've run ads promoting a particular brand (Acme Widgets--all models in stock--low prices, 2-3 day shipping), or targeting individual models (Acme Model XYZ. $1199.95 with free shipping--in your hands in 3 days). I've touted the five-star service, etc. I don't think I missed anything, but I never know. I look at what the competition does, and try what they're doing, too.
Part of the reason for the failure of advertising in the past was that my site didn't look polished enough. I didn't have a name, and I don't think the site met the required comfort level. I've had a ton of compliments on my new site, with people saying they think it's the best-looking site in my niche. I know it's going to take awhile to get known, but I wasn't expecting things to be this bad.