http://www.gnu.org/philosophy/who-does-that-server-really-se...
Even if the software was free, Microsoft could still spy on you. (I'm planning on writing a much more detailed response about how this relates to free software and SaaS later today; I'm stuck at work for now.)
Frankly it is because there is NO open source software in the VoIP/IM/etc space that is worth five minutes of your time, let alone full usage of.
Every time this point comes up the OSS apologists start naming some convoluted solution involving stringing five different software packages (and a SIP provider!) together to get even then a barely workable solution.
In general this is one of the least competitive sectors that exists. Right now we have Google+ and Skype, and even Google+ barely implements 1/8th of Skype's feature set.
No, he means free software. [0] [1]
[0]: https://en.wikipedia.org/wiki/Free_software
[1]: This confusion is why the term "open source" was invented.