In the end almost all of the tech successes and failures listed are reducible to a much simpler explanation: the effectiveness in using network effects to create moats, and defending those moats by adapting to technological change.
Controlling an operating system is a massive such moat, see Microsoft, Google and Apple. As is the first effective search engine - once you have both the eyeballs and the advertising revenue, it's almost impossible for any competitor to displace you.
On the other hand, an email service is not such a thing. Gmail is a good product but it's easy to duplicate or disrupt by other email providers and various messengers. A content hosting platform, be it for pictures, videos, or simple html websites, will not produce strong network effects by itself - this where the "social spin" you talk about becomes essential.
Yahoo persistently failed to create and defend these network effects. Messenger was such a strong network effect-inducing product, but they failed to adapt it to the mobile era and destroyed it through heavy advertising and blatant disregard for the user experience.
Facebook are currently in no such danger, they have a few massively addictive products built on strong network effects. Sure, some people might move to other forms of socialization - but billions more are going online, in countries with rapid economic growth. There is nothing on the horizon that significantly threatens Facebook's moat.
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