More people have Facebook accounts and use it more often.
Facebook and Skype are used for different things though. People use Facebook to send many short messages, see what their friends are doing, network, etc. The friends on most people's Facebook pages are good friends and acquaintances.
Skype is used to make phone calls, to talk live with close friends on camera. The conversations on Skype are longer because they're with close friends.
I think the only way they're the same is that they both have an instant messenger. I think more people use Facebook for instant messages because the list of people to message on Facebook includes close friends and not close friends.
However, I like using Skype more because it's easier to use.