A bad metaphor is bad in part because it misleads your subsequent thinking.
The metaphor of the "doubling penny" has been suggested recently to describe the growth of Google+ for example. This embodies some pretty deep misunderstandings about product adoption and spread of product and feature preferences in social networks and what sort of behavior one might expect to observe.
You can only adopt the product only once generally speaking. Your peer network (1st degree connections) similarly can only adopt the product only once. There is no doubling effect, there is not endless exponential growth of social networks or product adopters. There is flow across the network and spread of ideas between socially connected individuals but this phenomenon is not characterized by continual doubling (endless exponential growth) at all.
It is well known that product adoption life cycles follow a so called S-shaped or sigmoid curve. Rogers is the originator of the idea, but it was popularized here in Silicon Valley by Geoffrey Moore in his Crossing the Chasm etc. See http://www2.gsu.edu/~wwwitr/docs/diffusion/
In the case of free social media products such as G+ and Facebook we further have a scenario of essentially zero switching costs, and no requirement to choose one product definitively over another one. I post on multiple social networks, four networks today actually, and so do a lot of other people I know. In some cases people use software to aid them in this. The situation is not similar to other markets, i.e. for a consumable item, where I have to decide between two choices because I can only have only one or the other but not both.