ifb is an awesome thing, but there is no easy way of sharing it as far as I've seen. In Outlook you can set that in your own contact info and send your vcard, and it will be added in the receivers' contact lists. I've not seen an option to do the same from within zimbra, and indeed it seems that this field from outlook is not synced but maintained in outlook? It oughtn't really be hard since zimbra after all should know its own users' ifb urls, so it could even be generated automatically pending on a simple checkbox option. Also vCard (atleast Outlook's interpretation of it) has support for a FBURL field.