Don't use running clients as a guide. I am sure I'm not the only one who uses BTC without the client because it brings my netbook to it's knees when I try to run it. Instawallet works fine.
I am currently using a thin wallet for day to day operations and a paper wallet for cold storage. I hardly ever have the thin client running unless i need to make a transaction.
Number of hosts connected is not a good way of estimating number of bitcoin users.
Better way is to look at the average number of transactions I would guess and then do some math to figure out certain patterns in the frequency of spending, etc...
Either way the number of bitcoin users doesn't make a difference. It is the stability of the existing users that are important!