Does the bitcoin software show a count of how many computers are online and running it at any one time?
Any way to expand on that? For example, I don't run my BTC client all the time.
Still, that would be a useful metric in itself.
If that information is available, at least you could say something like "Right now there are 14,761 active wallets and 28 days ago at the same time of day there were 12,134" or whatever.
It would be similar to Skype saying "27,010,209 people online," which is what it says right now.