If you're part of that interest group, and you see the gap, then go on and fill it. You'll benefit yourself and others, wherever the opportunity leads you.
If not, then deep research is a must. Find the places (on line and real world) where people with that interest connect, and get evidence of frustration. That's not the easiest task if, as you suggest, people with that interest may not raise it much online. But you need to get to know your target market well, and connect with some insiders.
If the need seems real, and the gap seems real, then you can take encouragement from the vast number of specialist groups who have their own, many and varied online social spaces. You get to offer a great 2011 solution in your untapped market.
Then think hard about your minimum viable product. Maybe that's 'facebook lite.' Or maybe it's a facebook page. Or maybe it's a newsletter signup. Or maybe it's a blog. Or a ning group. Or a Q&A site.
The key tests for you are: can you reach people with this interest? And when you reach them, what's most effective in getting them onboard?
For what it's worth, my observation is that people with deep interests in specialist areas will value sites that they feel arise from that community very highly indeed. And for all its success in connecting people en masse facebook has consistently gone broad rather than deep in all but a very few categories.
That doesn't guarantee you success, but it does give you a flying start as you can view facebook as a great educator informing your target group of the value and methods of engaging online, so that when they find your more finely purposed space they will be confident in giving it a try.
Good luck, if you decide to back your hunch!