That's a broader definition than I usually see applied to "social network". By your definition, I was part of a half-dozen social networks (IM platforms) for years before any of the current "social networks" existed.
so by your definition any messenger is social network, any network carrier is actually social network, I think you got your definition wrong compared to what people understand under term social network
If people's understanding of social network is something similar Facebook, then this understanding is too confined. The key is to observe how people adopt the technology. My extended family are geographically dispersed. They have since formed a group using WeChat. Now they are sharing stuff in the group daily. I get to know them more than ever. This is absolutely functioning as a social network.
Do you teleconference your friends and family regularly? My guess is not. But it is always possible a new video app will comes up next year and gain wide adoption.
Today some number of people use Skype to video chat with friends and family regularly. So yes, Skype is also a social network.