Its got mostly to do with their closed platform. However, recently they have started opening up things (i.e. their codecs). Skype has established a phenomenal brand image that is under-utilized right now because its still chained to the computer. Nonetheless, Developments like the iphone app are a breath of fresh air and shows their strategy of truly mobilizing the app.
Imagine Skype integrating a money transfer layer into its application.
I would also think it is one of the biggest social network platform that has yet to be milked.