An important thing happened, and we call it Web 2.0. The new generation of services supporting networking, cooperation and higher levels of interaction made hierarchically structured knowledge repositories separated from authorial content obsolete. Today, the content became integrated with interaction to form even richer content. I think three types of services that did most of the kill: wikis, blogs and new types of user interaction. I wouldn’t put chats or instant messaging in the same category, because they were around since ever (f.i. IRC) and they leave nothing behind (at least publicly), making them useless for broader crowd and future generations.
Wikis are actually some type of a forum, where people gather content and knowledge. Instead of having knowledge scattered around in threads with comments and replies, all the users are working on the same "article", making it better and better as time and knowledge progresses. Few people know that wikis are not just Wikipedia (which is by the way a great example of human interaction achievement), a lot of companies use the same engine to build their knowledge base and web portals use them to build their web presence.
With Web 2.0, people went from anonymity behind nicknames to front row participation and ego building. Instead of participating in forums, millions of bloggers started making synthesis of useful forum threads to short and highly informative blog posts with an interesting side effect - building personal brands. Google’s page rank and other search engines did the rest, making forums less important and good blog posts better ranked and easily found.
New types of user interaction
There was a time when the only interactive thing you could do on the web was writing or replying (perhaps vote on a poll which had nothing to do with a content, at least technically). Today, you can post, view, like, dislike, support, comment, vote etc.. This fact gives users more flexibility on how involved they want to get with the content and the content becomes more informational. Knowledge is not hierarchically structured anymore, it’s rather scattered around the web in forms of multimedia (text, audio, video, etc.), with interaction activities attached to it (likes, comments, etc.). That makes it fun and more interesting, and if it’s good enough altogether, it will get synthesized and rebuilt into another form of information.
I think there is still space for forums on the web, but mainly in the functionality of huge chat rooms. On the other hand, general portals will probably be shifting more to combined approach, without the "Forum" link, but with integrated Web 2.0 services and approaches. This doesn’t have to mean using friends and connections, it can easily be the form of better support for different interactions and focus on participation of users around authorial content. Good news for users, bad news for portals.