If you have any application that adds buttons to the address book (like the contacts merger), you could have noticed that the buttons recently disappeared. This happened because of a bug in Monorail, the IM file transfer application.
Alban already fixed this bug and uploaded a new version to extras-devel. This new version also fixes other bugs, including a crash caused by the sharing plugin in Conboy.
Update: Note that you need to reboot or kill osso-addressbook after updating monorail to see the plugins again.