As of June 2015 Facebook decided to rework their API for 3d party plugins. Because of this “bad idea” possibilities of connecting to Facebook via Adium or Pidgin were disabled. But Facebook did not disable the possibility to use their service via Jabber’s XMPP protocol.
So instead of creating a default Facebook account within Adium or Pidgin, create an Jabber XMPP one and use the following login/connection:

<username>@chat.facebook.com

<username> is your official Facebook username (See Facebook webpage for details, Or go to your Facebook homepage and look at end of the URL)

[UPDATE]
There is something wrong with the SSL certificate creation and handling with Adium 1.5.10. So I used Adium 1.5.0 to create and store my Facebook.com SSL Certificate and upgrade to Adium 1.5.9 with still a working XMPP / Facebook Chat. So I think up to Adium 1.5.9 everything works just fine, when it comes to Facebook and jabber/xmpp (Get it HERE Src: https://trac.adium.im/wiki/PreviousReleases).