At some point the incumbent becomes so large and diversified that it can't be destroyed by a single competitor. Think IBM. You may come in and beat the incumbent company in the market that originally led to its success, but the incumbent still lives on in many other markets. The only way it is killed is by being consistently less fficient than the average entrant in many markets (i.e. Goliath falls, but there is no David to point to).
Facebook may reach that point before to long, although I'd guess they're not there yet.
IBM was beaten. They failed to evaluate the importance of the micro computer. Further, they failed to see the threat of micro computer-born CPUs to their business. The fact that IBM had enough momentum and clarividence to pivot into consulting is not relevant for this discussion. Today's IBM is not at all the IBM from the seventies.
Sounds kind of like how Kodak died out. It failed to anticipate the speed at which the market would shift into digital imaging and did a crappy job at entering other non-film markets (printers, cameras, etc). It's cash cow rapidly shrank to the point it couldn't fund viable entrants into new markets.
Facebook may reach that point before to long, although I'd guess they're not there yet.