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.