I own over 9500 domains (see my list at NameShopping.com), and only around 500 of them are developed. The rest are parked, where I make money from ads/links on them. But, the problem is that these type of domains don’t get listed well in the search engines, so they get very little traffic. I would estimate over 80% don’t even make enough money to cover the annual domain registration fee (I pay $8/year per .com domain). A few years ago I tried an experiment where I built what are called mini sites on 1500 of my domains that were making no almost no money from being parked (so I had nothing to lose). A mini site is basically a simple website created around the topic of the domain name, for the purpose of getting listed in the search engines. I created a somewhat automated system to create these pages, where I would choose a keyword for each domain (GetMortgages.com would for example use the keyword “mortgages”). I filled these pages with news about that topic from RSS feeds. For some sites I also added videos about that topic from Youtube.com. I even had some writers in India write informational type articles for a bunch of the sites (like GetMortgages.com would have an article about how to get a mortgage).
The results were that after a year, I was just making enough from these domains to cover the cost of the server ($80/month), so it was not worth the effort, and I changed them all back to being parked. I probably should have just kept the server running, to see if over the years the income would improve, but I didn’t, and from many articles I have read about mini sites, I am not sure it ever would have been worth it.
A few years later I then tried 400 of those same domains with a mini site type service named Evolanding.com, where they did pretty much the same thing as I had tried, but they did it much much better, and then split the income with me. Their pages looked very professional and were filled with news, videos, and affiliate offers. But, after around 6 months with them, the domains still made no more money than when I parked them, so I went back to parking them again. Evolanding has made a lot of improvements to their system recently so maybe the income would be higher now, but so far I have not heard any amazing results from any other domainers using them.