Newsgroups were far superior to web forums in many regards, but lacked glitz and editorial control, and hence fell to the wayside. With that said, web forums still beat Facebook's discussion plug-in abomination.
However, social network discussion systems, despite being so lousy, are appealing to many because they're so integrated with ease of posting without extra registration steps.
Other reasons for some web forums losing traffic / less activity is due to the forum software being overly-complex with too many features, and non-standardized compared to that of a social network discussion system.
Rambling on, but anyways, imho, web based forums will remain popular well into the future. But there will be much forum consolidation, and with many of them running on a social network discussion framework.
Ron