RandV
It's a wolf v2.0
Times change, and all it takes is not forecasting one popular development in your market accurately that drives enough of your customers away to fade into the limelight.
IBM once dominated the personal computer market...a market they've now exited. The term "IBM-clone" to describe computers compatible with software designed for IBM computers hasn't been used in 25 years.
Netscape used to be the leading web browser...they're also gone now after losing to Internet Explorer, which itself is now part of a competitive market with Google and Firefox and some other lesser browsers.
Blackberry invented the personal digital assistant, basically the hand-held computer. Apple invented the SmartPhone and swatted them out of any major share of the market like a fly (being sued over patent infringement on their device didn't help, but by the time they moved past that the public had moved past them).
Yahoo! used to be the leading search engine and news portal...they've just been sold after a couple years of negotiating and some embarassing failures in security, not to mention letting their product stagnate. Google rose up and stole their search engine market out of virtually nowhere. I still have a MyYahoo page, but I no longer use it for news, since their newsfeeds don't update very often anymore, and the news there is highly biased and doesn't give me much information on what's happening in the world.
MySpace used to be THE go-to to go to for social media, but they didn't innovate fast enough and faced stiff competition from newcomer Facebook, which kept developing new features users wanted. Now they basically exist as a way for bands that haven't kept up with technology to make their music accessible to potential fans, but those bands usually have Facebook pages now and don't update the old MySpace sites anymore.
You don't keep up, you fall to the wayside, and in this industry it tends to happen big. Maybe you still use Facebook and YouTube 10 years from now, or maybe they miss the boat at some point and become fading memories like so many others. You can't predict it, but don't say it's unimaginable...it's VERY imaginable.
I would make the case that your examples are companies or products that got in at the start or even innovated/created their specific industry. They're gone or minimalized now because the competition came in and someone else won.
I think it's much harder to imagine any of the big players losing now, at least unless/until it's replaced by a different technology. As things are now though, Facebook, Youtube, and now Twitter are integrated into nearly everything on the internet. They've also seamlessly transitioned from desktop PC to mobile, which could have been a big hurdle. So I really don't think you're going to get a competitor that does basically the same thing, like say vimeo, come in and knock youtube off the top and into obscurity. Especially when they're owned by Google.