Threadable (YC W14) now lets you opt-in to individual mailing list threads
blog.threadable.com
blog.threadable.com
Are you competing with Yahoo and Google?
Your website doesn't show how you're sustaining it.
I am a daily user of Yahoogroups and know several others who are in the same boat, therefore I naturally became curious seeing your service. I have been noticing a trend of Facebook Groups eating up Yahoo and Google's lunch, while the average Internet user slowly adopt Facebook as their primary communication tool.
We're sustaining Threadable by charging business users for business-specific features. Those features will actually be rolling out soon, and have to do with internal access control and integration with certain external services. We don't talk about that stuff on the site yet because it's still in beta, but expect to see it soon.
I definitely see Facebook groups growing in popularity, but we're a long way from getting rid of email. I think the reason Facebook Groups has been able to gain so much ground is because email-based products haven't really changed in a long time, and Facebook Groups provides a much better user experience. However, email is still the only place you can be sure to find everyone, and that's unlikely to change anytime soon, especially in business. Group communication products have a bunch of tiny network effects, one per group, and they succeed or fail on how well they can reach every team member.
I am not a business user but I look at these things as an average Internet user (and I am just generally interested). I believe Yahoogroups and Googlegroups are serving a specific purpose and need of average user. That is, to provide them with group-communication mechanism.
The reason why email is still popular and will remains so, is because email fills the basic and primary human need of direct (one to one) communication. The listserv on the other hand, also serve the basic and primary human need of group (one to many) communication.
I think the fundamental reason why email and listserv have stayed the way they have, is because they don't necessarily need much tinkering. So ultimately, it all comes down to where users get used to. We have adopted emails, listserv and now Facebook, as part of our communication platform evolution.
Facebook is an oddball because it supposedly does so much more (and there is lot of smokes and mirror on what exactly it does). The reason why I see people adopting Facebook Group (over Yahoo and Google) is because Yahoo and Google are probably unconsciously letting go of their hold on listserv needs of people.
But all of these companies are too big. A small or new company may come along attempting to disrupt the status quo. The problem as is I see it, is that chicken/egg thing. How can you raise yourself above Yahoo/Google/Facebook hold without disrupting the price model and still able to maintain yourself? That to me is a difficult question. You ultimately can't really win by having few users, even if they are dedicated. You need a whole lot of users.