Earlier quoted context omitted.
Skype's network effect and lock-in is extremely small compared to something like Facebook, or even something like an IM network. Think about it: early adopters (techies) start using a free competitor (or several free competitors). They get their less tech-savvy friends and family to switch. The rest of the world follows. It's easy to explain ("It's like Skype"), the carrot is obvious ("except it's free"), and the loc…
Last time I checked Skype's user count was above 500 million and they have 50+ million daily active users. Their network effect is pretty huge.
The only reason skype is the king right now is because nobody has motivation to switch to anything else. Skype works, without hassle. It's a good product.
The moment it ceases to be a good product, though, there is nothing holding them in their marketplace. The world can hop onto a new piece of p2p VOIP software in the blink of an eye.