It seems I just realized why Microsoft acquired Skype.
Skype is a social graph. Facebook has it, Google has it, and Microsoft ... now it has one too. Nowadays, being an Internet giant and not having a social graph is somewhat ... dangerous or something. Sogrut-s. :) The rest of the reasons discussed have a place, but, I think, secondary.
It should be noted that the graph of Skype is pretty high quality, half offline. The phone numbers that Google insistently solicited painstakingly collected in various ways, MS simply took and bought. Brilliant in a sense, the course.