To objectively estimate the number of users actually using Bitcoin, anyone can see from Blockchain link here
https://blockchain.info/wallet/#/ how many wallets are there now. Blockchain is largest Web wallet, so you can see actual number of Bitcon users - around 8 millions now.
I don't think the number of wallets on blockchain.info reflect the real number of users, some users create more than one wallet for security reasons, maybe have a lot of empty wallets and abandoned ones.
Number of transactions can be a smart way to see the growing.