I don't believe that xenForo is a reason ... or the forum competition at all. Most people prefer to use FB instead forums. And you know something? Not because FB is something so nice. For me is nothing. But is easier to use and (most important), easier to access it with smartphones. I'm seeing it everyday with my daughter and my niece. Don't know if it's a local service or international but the access 0.facebook.com which is free of mobile charges. All the day are connected there. The same person they connect to FB very rarelly from their laptops. So, the problem is how easy can access the news on nowadays.
Don't know how old are you but maybe you have heard for Usenet Newsgroups. Were so popular 15+ years ago. Now who knows them or who uses them?
I do have a feeling that in some years only some very big and/or unique forums will be alive. Maybe using vB, or xF or IPB has no sense. Should be few only.
We never know Chris, i will agree with half of your opinion. What if, if more members started to hate fb like me. To me, social community/sharing is like a cycle, if you don't match with the current trend you will be ignored. One day or the other people will be bored to use fb, they will find an alternative to spend their leisure time. They will join a forum that interests them & the no of people who likes & hates fb will be the same at one point. Thus forum's won't be dead at any point if you know understand your users needs.