Facebook uses asynchronous javascript calls (ajax) with responses in javascript object notation (JSON) over an HTTP connection (port 80).
Login is HTTP posted to https://login.facebook.com/login.php with POST
data md5pass=0&noerror=1&email={email}&pass={pass} where {email} and {pass}
are the user who is logging in's facebook email and password respectively. The
cookies that are set by the server must be sent during every POST and GET
request to the server(s) below
Every user on Facebook has it's own unique id (UID) number which can be found easiest in the URL of their profile eg "http://www.facebook.com/profile.php?id=1234567890" but can also be done via a search query to http://www.facebook.com/srch.php?nm={search termhere} and is stored in the cookie with name "c_user". Facebook chat uses these UID's for sending and receiving instant messages.
Visibility is updated by sending an HTTP POST to
htttp://apps.facebook.com/ajax/chat/settings.php with the POST data
visibility=true&post_form_id=1234 where post_form_id is an arbitrary id that
will be used throughout the process of communication (the facebook response
normally modifies the html that is contained within this form, and needs this
id to target it)
The buddy list is fetched via POST to
http://apps.facebook.com/ajax/presence/update.php with POST data
notifications=1&popped_out=false&force_render=true&buddy_list=1
Previous history of chat with another user is requested via POST to
http://apps.facebook.com/ajax/chat/history.php with POST data
id=596176850&post_form_id=1234 where id is set to the UID of the user you
wish to download history of
Messages are sent via HTTP POST to http://www.facebook.com/ajax/chat/send.php
with POST data
msg_text={message}&msg_id=3409501070&client_time={timestamp}&to={uid}&popped_out=true&num_tabs=1&post_form_id=1234
where {message} is the urlencoded text that is being sent, {timestamp} is the
local unix timestamp of the sender and {uid} is the UID of the user to send to
Messages are retreived by polling
http://{1-30}.channel{1-20}.facebook.com/x/{UID}/false/p_{yourUID}={requestnumber}&
using an HTTP GET request (not sure what the false bit is, but it can be true
too), where the {UID} is the uid of the buddy to download messages from and
{requestnumber} is an incrementing number, probably to work around
proxy/browser caches. The message that is returned from the server is like
for (;;);{"t":"msg","c":"p_800753867","ms":[{"type":"msg","msg":{"text":"i'm sending u a message","time":1209548203464,"clientTime":1209548202312,"msgID":"3409501070"},"from":800753867,"to":596176850,"from_name":"Eion Robb","to_name":"Jeremy Lawson","from_first_name":"Eion","to_first_name":"Jeremy"}]}
and are in JSON. Messages that are sent to a buddy are also returned back by the server.
Setting a status message is done by POSTing status={status_message_here} to
www.facebook.com/updatestatus.php
It has been announced that Facebook will expose an XMPP interface to Facebook's chat and presence.