Pretty different boats. Facebook/Skype requires a separate binary installation. Google Talk/Hangouts/etc currently requires Google's "Voice and Video plugin", which at least just integrates into Chrome rather than installing separate executables, but for the sake of this discussion, they're functionally equivalent.

Packing the necessary tools into a toolkit that a browser can utilize first-rate is, as you pointed out, a different challenge. Not sure what SIP or XMPP are "monstrous" though.