You may have noticed a "+ New" button show up on Skype recently. It seems unassuming, but it marks a pretty significant change in how the instant messaging software works. With it, you can message anyone via email, Facebook, WhatsApp, Facebook Messenger, or Twitter, and they can join the chat as a guest.
It's not full-on cross-platform integration as we've seen with services like Trillian, but that's likely impossible in an official capacity, so this is the next best thing.
See it in action in the video below, which features people dancing for some reason.