The newest feature is that for each person you invite that tries the service, you get an extra 5 minutes free international calling. So, with a large network, you can actually get free international calling! I believe that this is what makes this application something for more innovative than what we've seen so far on Facebook.
To make an application viral, having more friends using it, needs to add value to the person spreading it. So far, that value has been quiet intangible. With Reb Me, you actually get free calling for it!