1. Linux/Mac users would move because they had to.
2. Linux/Mac users that use Skype to communicate with long-distance friends and family would convince them to jump ship to another platform (if only to be able to do video conferencing).
3. People pissed off at MS would migrate on principle (though maybe only if there was a viable alternative to the way they currently use Skype).
I fear that it'll just go away, or actively get shut down by compatibility changes.
(My friends and I practically live on Skype. The one killer feature for us is persistent chatrooms; it's like IRC, except when I log in I get all the missing chat history I didn't get while I was logged out.)
What?
screen + irssi + /lastlog your_nick
2) I reboot to Windows to play games. There goes the screen session.
3) Skype on iPhones. Seriously. I ain't screwing around with a phone keyboard and terminal app to SSH in, screen -dR and scroll around just to get back into a chat.
This acquisition probably is the final nail in the coffin for Skype's hypothetical open-source Linux client.
I don't have high hopes at all that alternate versions of Skype will be kept up very long. Everything Microsoft does eventually leads back to Windows or Office.
I tried Wengo (now QuteCom) and couldn't get it to pick up the sound device or something like that. Maybe another PA bug, but PA has been a default feature of Linux desktops for 3-4 years or more by now. I don't remember the exact issue I had with it.
Pidgin isn't made for SIP/VoIP applications but I still haven't ever been able to get the gstreamer connector to respond correctly. The only IM client with which I've had success using video chat is Empathy, but it had a few issues too with closing devices/picking up subsequent calls.
Maybe it's just because I use Arch, but Skype is the only mostly-reliable video conferencing/SIP-like app on Linux that has actually been usable for me.