Well, I'm not a big fan of Skype but I tend to use it for longer calls with family overseas (US<->Europe) because I experience quite significantly better video/audio quality. Few months back (it's fixed now I think) I even had freezing video every so often on Google Hangout.
Or screen sharing, it was completely unusable in Google Hangouts on Linux just 2 months back whereas Skype didn't have any issue and worked (surprisingly!) flawlessly.
I don't really perceive the network effect that much, everybody I have on Skype also has a Google account.