To a certain degree(at least on PC themed forums) there is not much to be discussed anymore, performance leaps on both software and hardware levels are small to non existent, everyone probably knows everything about X86 computers by now that is publicly available. It's not the forum platform per se but it is the site's specialization that causes the declining interest. Upgrade-related questions by newcomers are overly repeating themselves. Creativity went down as well, not so long ago, overclocking, modding, tweaking etc were all things that were every time an unique and different creations. Today you just buy that in store because chinese do it cheap and purchases based on reviews will cause to everyone end up with same looking and same working rig.
Computers are old now, everyone has one and your grandma has three of them. At some point in the past, having a computer felt kind of unique because not everyone had it, now everyone has it. Computer culture did also span to Lan parties and internet cafes, which are extinct now because of home broadband and gaming laptops. And some of the computer games and social media sites became pathological addictions rather than tool for spare time entertainment.
To a bright side of things I definitely love my i5 with SSD and 24 inch screen hooked up to 100Mbps line instead of 13 inch CRT flickering at 60 hertz that I used with my 486DX2 desktop at the time and 9600 bit modem. There are no dial-ups and no night and day tariffs for dial-up lines and my printer does not weigh a ton!
Btw, reddit, twitter, facebook etc, these sites all have awful, slow and bugous interface... I still like traditional vBulletin or PHP bulletin board system much more.