Don't you love how she comes up with these diagrams!? LOL
I am not so surprised that Skype is bigger, I see the market as being different to the Facebook one, as the opportunities and technology are also different and more accessible to the parent and grandparent [even I understand great-grandparent] community. Facebook also provides problems/issues/challenges that for the majority are too complex; Skype is much cleaner in that respect. As the work Google has entered our lexicon, so too has Skype, "shall we skype?" not shall we phone each other.