Under what conditions can a socket be moved out of ESTABLISHED state?

Issue

• A system, such as a webserver or database server has a large number of ESTABLISHED connections such as these three examples:

# netstat | grep ESTAB
Proto Recv-Q Send-Q Local Address           Foreign Address         State      
tcp        0      0 unused:37926            vc-in-f103.1e100.:https ESTABLISHED
tcp        0      0 unused:47582            google.com:http ESTABLISHED
tcp        0      0 unused:40607            qc-in-f125.:xmpp-client ESTABLISHED

• Under what conditions can an ESTABLISHED socket be closed?