Facebook has the analytics information and wealth to buy any competitor.
IBM gave itself away to Microsoft when an exec handed Bill Gates a sweetheart deal for DOS.
Microsoft is still the dominant force in its original industry (desktop computing) which is still significant.
The Internet disrupted companies that weren't ready for it. Do you think something new is coming alomg to replace the Internet?
Even so, when one giant replaces another, so what? What problem does that solve? "When elephants fight, it is the grass that suffers."
Yes, this is the way of technology and it's happening faster and faster. One of the biggest reasons Microsoft ended up on its back foot in the 2000's was because it disregarded the threat of Linux, and woke up too late to the mobile revolution. In all these cases there's hubris mixed with inertia and when the landscape shifts these giants aren't paying attention.