Facebook and Twitter want you using their apps, so they aren't incentivized to play nicely with a concept like Hubs. Towards the end of Windows Phone 8.1 they opened it up to the apps themselves to implement Hub support, rather than Microsoft's team having to keep up to date with the API, but then again Facebook and Twitter don't have any incentive to support the Hubs that well (or even at all) when they really want your eyeballs directly in their own app.
The true death of the Hubs, though was the teams in question being handed over to the Skype brand and new features that would have gone to the hubs going instead to Skype-branded apps. It makes sense for Microsoft to attach a real brand to it, especially if they are making a play to bring more of those features cross-platform (which it sounds like is the goal: the Timeline stuff that's been announced is a differently organized, more app-centric approach to some of the Hubs; the People Hub is now on the Windows 10 taskbar, with deeper Skype integration; etc).
It's sad to see some defining features of Windows Phone 8 get broken up and parceled out into different apps, and there's definitely a question that if those things become way more cross-platform (including iOS and Android support) what becomes of Windows mobile, what is left for its unique and/or flagship features? (Other than maybe being a full Windows PC given the x86 on ARM rumors?)
I especially remember FB messages taking hours to arrive sometimes...at its fastest there would be a delay of several minutes. So much for "instant messaging". At least back then you could still access messages in the browser.
Blackberry also implemented the Hubs feature in its BB10 and Android devices, but again functionality is pretty limited, by design. IM is a big business, and the companies behind it aren't interested in your using them outside of their own app, as you'd be deprived of using "innovations" like Stories, GIF addons and emoji packs.