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Users. Skype is the replacement for MSN Messenger, which people just used for chatting with friends.

NAT / Firewalls cause a lot of issues. Peer to peer data isn't as straightforward as you'd expect.

The Skype people actually came from Kazaa, so they had a lot of experience with sending peer data across corporate / educational firewalls.

WebRTC is trying to generalize the technology, but it's new.

Plus asynchronous text chat.