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 think 8 millions is only an estimate since people can have am utiple wallets. Same with transactions. I think theres no specific way to evaluate bitcoin adoption rate but we can the the bitcoin volume being traded.