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 lock-in is minimal - download a new client, enter the contact you want to call, and you're done. There's no chat history, or complex social network, or tons of uploaded pictures, etc. etc., to keep people from switching.
I don't think the "mundanes" really use Skype, they still use regular phones. If someone's savvy enough to use Skype, they're savvy enough to switch to a free competitor if it's good enough.