Leave-taking online

Another Twitter-inspired post.  Someone just wrote:

Is it unreasonable to expect that chats should end with a farewell or sudden-unexpected-internet-fail? We say hi but never bye.

Makes me think of how difficult I find it to end a chat conversation--much worse than phone. How we 'take our leave' in conversation has been written about, but do the patterns remarked upon in that literature spill over into online conversations? How does politeness work in chat, especially with regard to ending or beginning conversations?