If Skype was part of a Microsoft social network, yes. Would you say you are inactive on Facebook if you use Facebook chat with colleagues? Facebook video conferencing?
If it was really the only thing I used, then yes, I would not count myself as an active user of a social network. It is OK to say google+ has N number of active users, what irks people is when they say it is the largest or second largest social network using that count
Indeed. It's not actually accurate to count Google+ as a separate product. They should be saying that Google has N number of active users.