Skype is a telecommunications application software product that specializes in providing video chat and voice calls between computers, tablets, mobile devices, the Xbox One console, and smartwatches via the Internet and to regular telephones. Skype additionally provides instant messaging services. Users may transmit both text and video messages, and may exchange digital documents such as images, text, and video. Skype allows video conference calls. Skype implements a freemium business model. Much of the service is free, but Skype Credit or a subscription is required to call a landline or a mobile phone number. At the end of 2010, there were over 660 million worldwide users, with over 300 million estimated active each month as of August 2015. At one point in February 2012, there were 34 million users concurrently online on Skype. First released in August 2003, Skype was created by the Swede Niklas Zennström and the Dane Janus Friis, in cooperation with Ahti Heinla, Priit Kasesalu, and Jaan Tallinn, Estonians who developed the backend that was also used in the music-sharing application Kazaa. In September 2005, eBay acquired Skype for $2.6 billion. In September 2009, Silver Lake, Andreessen Horowitz, and the Canada Pension Plan Investment Board announced the acquisition of 65% of Skype for $1.9 billion from eBay, which attributed to the enterprise a market value of $2.92 billion. Microsoft bought Skype in May 2011 for $8.5 billion. Skype division headquarters are in Luxembourg, but most of the development team and 44% of all the division's employees are still situated in Tallinn and Tartu, Estonia. Skype allows users to communicate over the Internet by voice using a microphone, by video using a webcam, and by instant messaging. Skype-to-Skype calls to other users are free of charge, while calls to landline telephones and mobile phones (over traditional telephone networks) are charged via a debit-based user account system called Skype Credit. Some network administrators have banned Skype on corporate, government, home, and education networks, citing such reasons as inappropriate usage of resources, excessive bandwidth usage and security concerns. Skype originally featured a hybrid peer-to-peer and client–server system. Skype has been powered entirely by Microsoft-operated supernodes since May 2012. The 2013 mass surveillance disclosures revealed that Microsoft had granted intelligence agencies unfettered access to supernodes and Skype communication content. Throughout 2016 and 2017, Microsoft redesigned its Skype clients in a way that transitioned Skype from peer-to-peer service to a centralized Azure service and adjusted the user interfaces of apps to make text-based messaging more prominent than voice calling. Skype for Windows, iOS, Android, Mac and Linux received significant, visible overhauls.