I used to be real big on micro-niche mini-sites, and they were pretty profitable for me, I was averaging over $25 per month per site, and figuring each one cost me $10 for the domain and about 3.5 hours to put up and promote, that was a decent income once I had enough of them online.
The problem for me was it became crazy-mad to keep organized. At the height of it I had over 170 of them running, and just staying on top of the accounting and domain renewals began to feel like a boring full-time job, so I started culling my inventory down.
I think they're still a good money maker if done right, and I don't think the search engines are specifically weeding them out, you just have to set them up properly so they don't appear as thin affiliate sites or simple MFA sites.
Make sure you have About, Privacy, Contact pages on them with descriptive and unique info. I even add a Press page because I always used at least one press release to boost a new mini-site, so I'd republish it on my Press page. The setup makes your site look legit or not.
As for content pages, I had some mini-sites with 1 page, and some with up to a dozen pages, it depends on your niche and what you're offering on the site. For instance, I liked to build sites around calculators of some kind, like an interest calculator tool site, or a BMI (body mass index) calculator site, because you'll pick up lots of free links by offering a tool on your mini-site, and won't need much content to fill your pages either. Also, even if you can't script a tool, you can usually find one at hotscripts or somewhere else to use on your site for free.
As for promoting mini-sites, I would go to Yahoo Answers and find 5 questions related to the topic of my site, then I would turn each question into a "How To" or "Did You Know" type headline. For instance, if I found a question like "what's the best way to estimate the interest on a loan?" I would make a headline from that of "How To Estimate Loan Interest Rates"
Then I'd turn each of the 5 headlines into short (300 words) articles and submit them to various article directories. If I was lucky, the 5 questions I'd pick on Yahoo would have answers already, which I'd use as the research for writing my articles.
Then I'd use Open Office, add a few stock images to each article, export each one as a PDF and submit them to doc sharing sites.
Again with Open Office, I'd create a small presentation slideshow using the 5 headlines and a couple of bullet points for each one and upload that to doc sharing sites
Using the slideshow I'd created, I'd record myself reading the headlines and bullet points and turn it into a short video to upload to video sites
Use the audio recording from my video as a podcast I'd submit it to podcast directories
Combine the 5 PDF's I made earlier into one PDF, add a cover page and Index page with a table of contents, then upload that to ebook directories
And finally, I'd issue a press release announcing the site as a helper for people with whatever problem the site was aimed at helping with, and like I said I'd also publish my press release on my own Press page.
It seems like a lot of work, but it's really about reusing the same 5 headlines and content over and over again in different formats. I had it down to taking just about 3.5 hours for everything.
Then I'd just social bookmark everything, grab the RSS feeds from all the places I'd published or shared content and blast them to ping services and with RSS Submit.
I mostly promoted affiliate products or services with mini-sites rather than using AdSense. It seemed to average out better for me that way. I'd also use them for list building because the traffic was highly targeted, so if I could find multiple affiliate offers to give people then I wanted to collect their email addresses to hammer with future offers. The signup rates were low, but again highly targeted so I'd make a few sales from future email blasts too.
Anyway, hope some of that's helpful to someone.
