Skype is also a mess right now; with chat application bugs, UX issues (why does the freakin' mute button move around) calls that can't be joined or start cyloning, participants are suddenly dropped mid call. Throw in the spying allegations ...
Hangouts is great for video but is a really clunky experience otherwise.
With everbody dropping the ball, there certainly is room for Slack to come in and take control of this space.