The problem
After upgrading from 12.10. to 13.04. empathy doesn't show online contacts anymore. All accounts seem to be affected by this (facebook, ICQ). Sporadically a contact is listed as online for a few moments but then disappears again (I guess these are contacts that just went online). There is at max one contact listed as online at any time.
What I tried
I tried removing and adding the accounts but had no luck. I also tried removing cache and config files of empathy and telepathy. Again no luck. Running empathy debugger did not reveal any problems.
UPDATE 1
When removing and readding my facebook account I now get a crash in telepathy-gabble: telepathy-gabble crashed with SIGSEGV in dbus_message_get_reply_serial()
Upon continuing after the crash, the problem remains (no contacts shown as online).
UPDATE 2
There is at max one contact listed as online at any time.
Sometimes more than one contact is listed as online. But only ever for a short period of time.
UPDATE 3
When starting empathy with facebook account configured:
empathy-auth-client crashed with SIGSEGV in ag_auth_data_get_parameters()