Someone inquired why one would choose to use a forum over a chat medium such as MSN.
Regarding public discussion
Forums encourage more people to engage in discussion, by exposing the topic of conversation to a wider audience than MSN could target. Allowing for the use of group chats, each person must still be invited to take part.
IRC is a bit more suited to public discussion for similar reasons, but its realtime interaction and not the persistent nature of the web leads to rapid topic decay. With everything happening live, a given topic may last for a few hours at best on any instant chat medium.
On the web however, a solid topic may generate weeks of discussion, if not more, as users can return to a topic at any time, when it is convenient for them to do so. Likewise, new users may jump into an existing discussion, and revive a topic that has not been active in days.
Regarding private discussion
MSN requires users to use specific software (while there are alternatives to Windows Live Messenger, such as Pidgin, these clients often lack all the features of the ‘official’ ones).
Including chat protocols other than MSN, such as ICQ/AIM, Yahoo Messenger, Jabber, or IRC only further widens the rift. If one person uses MSN, and other uses AIM, it inconveniences one or more of them with the process of creating yet another account for some service, and burdens them with the overhead of one more application running on their system.
For this reason above all else, forums are superior to IM.