Ways To Go Viral

The formula is given without explanation. You can rederive it without too much trouble.

You can't use the common formula for compound interest because users are assumed to send invites only once. Compound interest would assume they were sending invites continuously over the cycles.

If you look at the number of users added each cycle (the top row) you can see that it doubles each time. It's given by c0 times K^i, where c0 is the initial number of customers, K is the virality coefficient (2 in this case), and i is the i-th cycle.

Adding each of the terms up to i to the original c0 gives the total number of customers after i cycles. So you get a sum:

  sum over i from 0 to N of (c0 * K^i)

which using an exponential sum formula (http://mathworld.wolfram.com/ExponentialSumFormulas.html) gives:

  c0 * ((1-K^(N+1)) / (1 - K))

Multiply by -1/-1

  c0 * ((K^(N+1) - 1) / (K - 1))

and N is the number of cycles (given by t/ct in the slides)

  c0 * ((K^(t/ct+1) - 1) / (K - 1))

> A more pervasive example is the “Sent from my iPhone/iPad” signature at the bottom of every email you send from your iPhone or iPad. Yes, even Apple is using viral tactics (actually, I believe BlackBerry started that with “Sent using BlackBerry”).

As noted in "Founders at Work", Hotmail used this tactic with a lot of success. I don't know if they were the first to try it, but they were definitely the first widespread example.

Am I wrong in thinking that G+ attempts all of these things and yet still struggles, or am I in the wrong circles? Or is it that Google has yet to reach a tipping point with their product?

It was just a matter of time before the "Number Posts" made it to Techcrunch. I am sure slide shows are not a far way off.