One of the most famous examples of virality is YouTube. Before the site became the huge destination it is today, you would most likely find a YouTube video embedded on news site or personal blog. You'd watch it, and at the end, you'd be invited to email the video to people you know, and you'd also be given code to embed the video on your own site.

true viral growth is incredibly rare. It took me a while to appreciate this: very few products have sustained a viral factor over 1 for any meaningful period of time.

for a consumer internet product, a sustainable viral factor of 0.15 to 0.25 is good, 0.4 is great, and around 0.7 is outstanding.

When the viral factor is less than one, it can interpreted as the amplification factor a = 1/(1-v). To calculate the total number of users, multiply the number of users acquired through non-viral channels by the amplification factor