Missing the point
Most of the previous commenters have missed the point. Skype were free to use, or not use, the GPL.
They chose to use it. Consequently they got the code free of charge. However they were bound to distribute the source with the application. This is still cheaper than paying for code.
The relevant section of GPL v2, which is probably what applies here,states:
3. You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following:
a) Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
b) Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
c) Accompany it with the information you received as to the offer to distribute corresponding source code. (This alternative is allowed only for noncommercial distribution and only if you received the program in object code or executable form with such an offer, in accord with Subsection b above.)
Anybody buying closed source software is usually bound by an onerous licence. Why should the GPL attract such negative comments? Are people expecting a totally free lunch?