So it would be better to frame your question in a way which got the results in which you are actually interested.
Do you want to know which niches are being served by HN users? Ask. Do you want to know how to use SEO to turn your 10k net profit business into a 50k net profit business? Ask. Do you want to know if the majority of HN is starving wanna-bes? Ask.
This question in an exceptionally poor proxy for your real question.
The hardest thing for me isn't spotting or monetising the opportunity, but in balancing everything for a reasonable quality of life without burning myself out. That and discovering and handling the minefield that is running your own company and figuring our the finances, taxing, invoicing and asset management.
My company does community websites, forums and the like. I chose to start with areas I'm most interested in, but have avoided areas where there isn't an after-sales market. I aim making the lions-share of the revenue from affiliate schemes such as the eBay Partner Network, Amazon, and then Affiliate Window has a lot of companies running schemes specific to the communities I'm running.
I don't push the affiliates... I just aim to build the communities up to the point that enough conversation produces the links to used or new items that then produces revenue.
Once a community is successful, I look for another.
What I've discovered is that revenue earnings relate to purchase price of items, and that you can make significant money in high-value items sold through eBay. I would now recommend people look at cars, motor-homes, boats, etc sold through eBay, and then build affiliate based communities around those. Which means building communities who love and use those things, and affiliate earnings is a natural by-product of letting those people communicate.
There is a lot of money in affiliates with a low entry barrier so long as you can attract the people who will click on the links, and if you happen to have also attracted people who would post the links (you can auto-rewrite server-side to include the affiliate element) then you have a winner.
So that's what I do in my spare time (30 minutes per day), and it's wildly successful given the effort invested (I'm using off-the-shelf forum software).
I could even make this my job full-time, except I personally need more stimulus than just running a farm of community forums.
Also... beware the politics. If you can stick to a same-sex topic where people are unlikely to sleep with each other, you'll get a lot less politics.
I did a music forum that was pretty successful, but a third of people slept with each other and then bore grudges for the experience (it must've been bad) and that spilled back on to the site.
You don't want to spend your time trying getting adults to behave like adults (though technically that was the problem itself).
In two parts this simply means:
1) You had better be interested in the topic
2) Check which other sites already exist and position yourself accordingly
My most successful forum is LFGSS http://www.lfgss.com/ , which is London Fixed-gear and Single-speed... a cycling forum which luckily caught a trend before it got big. The positioning on that was that whilst other cycling forums do exist, others failed to recognise that cycling is a local activity and that cycling itself splits into various tribes and that even local activities only have appeal to one tribe. I love cycling, and I was riding fixed, so once I recognised that I could just start the London forum I was pretty much done.
To launch it I just went on to sites like bikeforums.net and announced in the most applicable place that I was starting something I hoped would appeal to the niche I'd highlighted. About 15 people signed up on day 1, just enough to have conversations between each other... and with about an hour a day invested in chatting and going on the odd ride I get it to the point of getting 30 > 50 new users every day and am now somewhere over 25,000 members with over half of them actively using the site each month and a unique visitor count (Google Analytics) of 250,000 per month.
My other forums aren't doing as well, but all produce revenue and the platform is already paid for by LFGSS.
It took me a while to work out the revenue model (get users first!), but once worked out they're doing fine.
Seriously though... I would look at eBay and find high value items (above $1,000 USD average) and ask myself which of those is something I'm interested in and could invest in for the near future (as in... treat as a hobby). Then create the site, and instead of thinking about the revenue and site, become a user and just contribute as you would if you were a user. The money follows that.
I assumed this was the point of the question because it focuses on net profit but you're right, there's not much to add to the answer that could be helpful to others. Also, I answered as a single founder which is different than a team of people saying they just cleared 50k/year from a net profit per person perspective. But because of the way the question was phrased, I'm not sure if that matters here.