I think you might be splitting hairs with market segments here. It’s correct to distinguish skype as a communication tool and not a social network; however the social network category is a subset of the larger communication tool Market! What we’re seeing here is not a mismatch of markets but a completion between its subsets.
I would (over simply) say there are three major subsets here (online), Social networks, direct communication, and forum/bb. They are all part of the same market, all competing for the same audience: And while you can, and probably should use all three some prefer to limit the channels of communication they engage in.
I would propose that most people have a preferred channel. Or at least an order of preference to how they wish to communicate with others (mine being 1.Direct 2.Forum 3.SN). What Skype is doing is attempting to change the consumers channel preference (differentiating) by saying what they do better (phone/video) is more worth your time and investment, than say status, and profile browsing.
It is incorrect to say their assertion is right or wrong, but it is fair to say that this assertion is right or wrong for certain people. Skype is trying to communicate to those who agree (with them) that they would rather “stay in touch” using a tool like Skype by highlighting a limitation of another tool, social networks.
Obligatory flawed analogy:
Ford can sell cars by selling a family road trip cross country instead of taking a delta flight. You can argue they are not competing in the same markets (car sales vs. airline tickets). But if you acknowledge that their together in a greater market context (transportation) and that the ad is highlighting the difference in experience of that travel the ad is very valid. Their not bashing their differentiating.
While calling it a 'Tax' is effective in getting across how distasteful this is; I think 'Ransom' is far better word. After all the idea (ideally) of a tax is to pull money together for larger efforts. Where as this is clearly a group of individuals who are taking the money for there own purposes.
It's semantics, but calling it ransom takes away the possibly perceived 'legitimacy' that Tax can have as a tool of the state. Paid ransom can be seen as a necessity; however it always leaves a bad impression of the one demanding it.