Viral Pockets. However, you can see exponential growth with an average k-factor less than one.
I've seen it happen several times now, e.g. in my last company and now in two of my portfolio companies. This phenomenon is due to what I call "viral pockets. " These are subgroups of your users that are experiencing exponential growth, i.e. pockets of virality. The overall average k-factor is still less than one, but your service is nevertheless growing at a rapid rate fueled by internal dynamics. The takeaway is that you should be looking at subgroup statistics to spot these pockets as well as trying to engineer them via subgroup landing pages, emails and other particulars of your viral loop. 45 idées pour générer du Buzz.