Pet owners are another example. Apparently some of those "facebook for dogs" sites are doing pretty well, since the dog owners who use them tend to be middle-class and middle-aged - a huge market that most tech startups ignore. Think different!
Ancestry.com is a good example of the opposite, though potentially very lucrative function: small quantity x high price. The site has only a few million active users, but each of them pays a decent chunk of change per month. Those users also use the site very frequently, and for long periods at a time. (Again, contrast this with a service like Facebook: lots of users, each one presumably browsing or using in very quick and sporadic bursts).
People forget that a market involves both # of users -and- willingness to pay. There's more than one way to reach a large addressable market.
They are traditionally backed by large companies, but a talented/knowledgable/passionate group can shake things up in a big way (with stable/long term revenue streams).
Interesting to observe the "what is your passive income" type threads and realize just how many in this community are unaware/under-appreciate some of these amazing business models. Not shocking to see medium sized 1-man projects to have lifetime revenues over 1 million.
One of my first gigs was at one of these types of companies. While the engineering was deplorable (one reason I left), the insight into the business side of things has forever changed my path.
Not sure how old you are, but I'm 24 and have a premium Ancestry.com account. My family tree is really strange, so tracking through it is interesting to me.
Ancestry.com for pets.
Gonna be HUGE.