Facebook now also let you respond to some posts on your profile by email. Their servers then work out which bit is your comment and add it to the “conversation” on your profile. This is very useful, but if we had this for talkphysics.org… what is a talkphysics.org forum for?
For me, a combination of email and forum works well. Google Groups work like that (and Yahoo Groups to some extent). You can reply by email or on the website, but the big advantage is that whatever method you choose, the website keeps the conversations in “threads” so that you can find what you are looking for later.