Google doesn't release Hangouts for WP (Android and iOS have it and BB10 can talk to Hangouts via Gtalk integration)
BB doesn't release BBM for WP (Android and iOS and BB10 all will have it soon)
Viber updates WP and Android apps with new features and releases a desktop client (but WP users have to wait for "next 3 months")
Whatsapp leaves WP client as in the dust with their other versions (including BB10 versions)
Facebook has Messenger apps for Android and iOS and for legacy BB but nothing for WP (integrated FB chat is a joke)
Microsoft releases Skype for everyone (even Ubuntu) and Skype actually performs better than on WP on some other platforms. But even in its best incarnation, sorely lacks in mobile specific messaging features compared to everyone else
Is it just me or is this a critical thing MS is missing coz messaging works entirely on network effect and not on merit of messaging clients. They better fix this quick coz right now, no matter who wins the messaging wars, MS loses