Simon Willison’s Weblog

Why does Facebook chat use subdomains so aggressively?

10th January 2011

My answer to Why does Facebook chat use subdomains so aggressively? on Quora

Probably because it involves long-running connections. Browsers have a limit on the number of connections you can have open to the same domain at the same time (I think it’s 8 in most browsers these days). If Facebook chat opened a connection to www.facebook.com and you opened up 8 Facebook windows you would no longer be able to navigate to any more Facebook pages, since all 8 connections would be taken up by the long lived chat connections. By connecting to a different subdomain for each connection this problem can be avoided.