As you stated, optional services. Skype itself is free. It can be used without any of the services you mention.

>Not quite so. Over time I subscribed to several optional services which go with Skype. But I'm not doing that ever again - because of the predatory techniques they apply. I have set my skype-to-phone to automatically recharge from my card when it falls below 5$. Then, independently of this, I paid 50$ for premium membership, because it was the only way back then to get group video (my family lives in three time zones, so we need that). Next year, it was someone else's turn to pay, so I didn't - but Microsoft thought otherwise. They have set it on autorecharge. I have cancelled that within minutes, sent them an angry note, and then the next year the same thing happened again.
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>Still, I don't think they are getting that much revenue from such services. So, if they aren't getting money from it, why are they keeping that service alive? The answer you won't like is that they more than make up the cost of running skype in sales of (aggregated?) data acquired through it.