That is the question I've been asking myself lately. Mailing lists used to be a staple communication mechanism on the Internet. Now nearly all of the mailing lists I'm subscribed to are very quiet - still have 10,000+ subscribers on most, just no one seems to use them. The distinct trend I am noticing is that people are forgoing mailing lists and using quick question and answer sites like Yahoo! Answers and StackOverflow to get the answer to their questions. (Or using the Facebook commenting system or Twitter - but that depends on your friends, connections, and followers). Experts Exchange used to hold the position and used to do quite well but then shot themselves in the foot by putting annoying barriers in the way. People went back to mailing lists after that fiasco.
Unfortunately, there are two significant problems with the Q&A websites out there that mailing lists solve and StackOverflow is demonstrating the problems quite well.
The first problem is community and ownership. On a mailing list, anonymity is possible but you don't get to be a community leader let alone the mailing list owner by being anonymous. About half of the StackOverflow questions get shut down before they get started by moderators who hide behind rather anonymous-sounding usernames. Yahoo! Answers would have suffered a similar fate had spammers not found it to be an effective medium. StackOverflow/StackExchange is experiencing growing pains by leaps and bounds but is suffering the Wikipedia effect, where there is significant power in the hands of few people who aren't vetted very well and repeatedly show up to cause long-term damage to the site. The current set of SO moderators police the site and abuse their power by shutting down valid questions after only being up for a few minutes. On the other side of things, mailing lists are generally open to everyone to ask questions and any moderation queues are used to just filter spam from reaching list members. The current SO strategy will ultimately kill the site in the long run and significant damage has already been done, but most people don't realize it yet. There is also the question of who owns the site. Ownership is important because it creates the important hierarchy of accountability. StackOverflow, from the observer perspective, appears to have no owners, which makes it seem like a free-for-all website. That also causes problems that are a lot harder to pin down in a single sentence but suffice it to say that where obvious ownership exists, chaos, which always exists, is better kept under control.
The second problem is continuity and continual learning. StackOverflow, Yahoo! Answers, forums, and other mediums are hit-and-run. You ask your question, you get your answer from someone, you give them karma/points/whatever, and you go away and generally forget that there are other people who need help. These sites imbue selfishness. Whereas a mailing list is a continual stream of thoughts - there are regulars but other people help out too as part of a continual community effort to improve each other. Everyone picks up tidbits of information here and there and refines knowledge in a common area as well as occasionally replying to posts, which further contributes to the stream of thoughts.
Now I'm going to give one downside to mailing lists that StackOverflow really excels at: SO excels at bringing together a collection of strategies for software development and selecting the best approach at the point in time that the answer is selected. It is basically a Wikipedia for common software development questions and answers and, specifically, produces a set of best practices that are impossible to obtain elsewhere in a single location. Which is why the site, if it doesn't change, will suffer the same fate as Wikipedia, only it is more deviously hidden. In fact, we can already see SO turning into a Wiki where the answers can be edited by other people.
This puts me in a bit of a dilemma: Should we use mailing lists for our questions and answers? Should we use StackOverflow/StackExchange/Yahoo! Answers/forums? Twitter/Facebook? It would be nice if we could somehow have the best of all of these worlds. This seems to be what we have been trying to strive for over the years of IRC, mailing lists, and Q&A websites: Hey, I asked a question that someone else may have gotten a great answer for, so I should use that, but I also want the personal touch rather than "Closed as Duplicate/Too Localized/etc." without any interaction by those closing the question. (Side note - "Too Localized" is irritating because it comes off as "You asked a Dumb Question, go away" - imagine how that would make you feel being the recipient of that.) Closure of questions is the equivalent of "This conversation is over because we decided it is over and there is no disputing our decision." Humans have emotions and desire interaction. Therefore, canned responses and question closures are too stoic and drive people away. Okay, so you've repeated yourself a zillion times already and it is kind of boring to do it again, but to someone else, it is that direct response that says, "You are important so I won't brush you off as a nobody."
If we can achieve the above while simultaneously having an effective database of best practices, it won't matter if mailing lists died. I'll be sad that I can't simply use my desktop e-mail client for community communication, but I'll move on too.