Yep, cell phones and Facebook killed all the forums.
One thing he didn't mention that also played a big part was all of the forums being bought up by companies like VerticalScope. They roll in, change the local template that everyone is familiar with to a shitty one they use across-the-board, kill the local community, and then try to monetize every part of the forum. Being bought out by one of the large media companies is pretty much a guaranteed death sentence for any forum - motorcycle or otherwise.
Personally I hate surfing on my phone and do it as little as possible. I don't even have my CMR account on it. It's an absolutely terrible device for displaying information. The PC Master Race is a thing.
There has been a small resurgence in forums lately and I think a lot of it is due to people souring on big social media.