Last Friday, by coincidence I came accross the Firefox companion for LinkedIn. I decided to try it out because I am more and more in my Gmail account in the browser and less and less in my Outlook app on my desktop. Something strange happened. In my mailbox, I was now able to see a LinkedIn button next to the name of people who emailed me. And I noticed that I click these buttons all the time because I want to know if they use LinkedIn, how many contacts they have, what their profiles say, and if it is someone I know if we are already connected. In two instances it actually made me sent a connection request to other LinkedIn users. It is really amazing to actually see and realize how sticky this feature is and how it grows the use of the service. In my humble opinion a very good example of a service provider using viral marketing to its advantage.