Speaking at BUILD Satya Nadella has announced that Skype has more than 300 million monthly active users.
This makes it a pretty large communication network, but makes it much smaller than WhatsApp and Facebook Messenger for example, which can both make VOIP calls and have more than a billion users.
Of course Skype is much more likely to be used for voice calls, and we do not know how many WhatsApp messages end up being calls. Microsoft is however working to expand Skype as a messaging platform much like Facebook Messenger, integrating Cortana in the chat interface, suggesting Microsoft will soon be going after the same customers. This is also supported by Microsoft breaking up the app in Windows 10 into 2 separate messaging and video calling apps, though the company has reversed direction recently by announcing a coming full-featured UWP application shown above.
Microsoft also showed off Skype for Hololens.
Keep an eye on the site for more BUILD news.