I just tried it. It sounded like a very neat idea.

But it didn't really worked. It wasn't able to connect a single contact with its Facebook profile. In one case I checked more details: It seems that he used another mail address in Facebook than I got from him via GMail. So in GMail, I added this other mail adress to the contact in hope that Rapportive could now connect it. But it still couldn't -- it only seemed to check the mail address of the mail but not the other possible addresses of this contact.

What I always missed was some way to manually connect it (in a way of course that it would remind/save that).