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))

Thanks for expanding on that kcl. I wish I could go into that depth in the article :)

> 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.

Thanks for pointing to this. Wasn't familiar with this one but it is a great example of using the "signature" tactic.

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?

I'd claim that the relative success they have so far is a result of them using these tactics.

Good point. There is a simple formula to measure it, but there is no easy formula to get.

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.

This one seems OK though -- he's attempting a taxonomy and does a reasonable job. They just made the headline sound like it was a fill-in-the-blanks top ten list.