I'm trying to be cautious with the information I post, because I don't want it to be perceived as advertising.
Your perception is a little off, but I can see how you got there. We have 46 websites of our own that we have been building for roughly 13 years, and they are all doing well; building communities isn't the issue there.
A few years ago, though, I stumbled across a site that had a very inferior message board system, with very little revenue being generated, in spite of a lot of traffic. The owner of the site was being hit with increased hosting fees, etc, to the point that it cost more to keep the site going than he was making. Worse, he had little technical expertise, and just changing servers was over his head. He had a good idea originally, he just didn't know how to make it profitable or how to manage the technical side of it.
So, I approached him and offered to join forces; I would supply him with an updated website (for free), unlimited hosting (for free), I would handle the marketing for the site, and in his case, I offered to take over moderating (since it was becoming a full time job for him). In exchange, I wanted to use my own ad network instead of what he was using, to increase the revenue, and then we would share the revenue.
He agreed, and within about 3 months we had everything rebuilt and working smoothly. From the day we joined forces, the revenue for the site increased by about 350%, so his share was already 75% more than he was getting before.
Since then, we've partnered with 5 other sites in a similar way (4 of them still do their own moderating, but we handle the design, development, marketing, and ads). It's worked out well for everyone, which is why I'm now working towards expanding to more partners.
I also bought one site out entirely, which wasn't really part of the plan, but that was his preference. It was a bad choice for him, though; in 4 months, we've already made more in ad revenue than the purchase price.
So, yeah, I get why you would think that this is somewhat parasitic, and that might be a handicap worth considering; I simply hadn't thought of it that way. I compare us more to an ad network; sure, they get 40%, but the 60% you get is still a lot more than you would have gotten on your own.