Yes, this is a bug in libosso-abook triggered by Monorail's plugin for abook. When Monorail's get_menu_entries() returns NULL, the address book stop checking the other plugins (contacts-merger). And a critical appears in the logs:

GLIB CRITICAL ** default - append_menu_extension_entries: assertion `NULL != menu_entries' failed

I wrote a workaround in Monorail abook-funsharing/abook-funsharing-extension.c get_menu_entries() and it works fine.

Meanwhile, 'contacts-merger' can always be started from the console.