I m not sure any way is here to determine this. if we think that we can determine this, so it is time taking and also it has too much difficulties here.
But i m sure the number of the people are growing up who using the bitcoins because 2016 is better for bitcoins. we can guess about it because bitcoin price is going up and also changes here in it, that's all prove that people have take interest in bitcoin and using it mostly.

Bitcoin process about million transactions every week currently. I think there are up to 1 million users who use Bitcoin regularly, and definitively well over 10 millions who tried Bitcoin at least once or just bought and holding coins - blockchain.info alone has about this number of accounts.

Bitcointalk is the largest bitcoin forum. Please correct me if I'm wrong.
So if we can have a idea how many peoples are using this forum than we can get an idea at least.

Maybe it will be 40-50% less from the actual number.

I don't think there is a way to measure the number of people who uses Bitcoins and if there will be like a survey of how many is using, I don't think one will admit that he or she is using Bitcoins regardless if active or not since not everyone who holds Bitcoins are actively spending it, they are keeping it for profits.

These numbers look close to what I think too. Although the number of accounts on blockchain.info doesn't give us the exact number of users, we can have an idea about it looking there. So I think the number of users is from 1 to 2 million.

I don't think it's possible to measure the number of people using Bitcoin.

Well I don't think it's really possible to know the exact number of people. However since using Bitcoin means a person needs to have at least one address, you can try and work out a very rough estimate by looking at the total number of addresses that hold a value here: http://www.bitcoinrichlist.com/charts/bitcoin-distribution-by-address?atblock=410000
and the total number of unique addresses here: https://blockchain.info/charts/n-unique-addresses

I think bitcoin exchanges can provide somewhat close figures as to the number of actual bitcoin users.Most bitcoin users have bought or sold their coins at some point of time at these exchanges

Like most people said, its very hard to determinate how much people are using btc.
"More than 25 million addresses have been referenced in those 31 million transactions. The total number of addresses, of course, gives an upper limit for the number of possible bitcoin users. Naturally, not every address ever referenced is still in use. In fact, it turns out that less than 10 percent actually have any bitcoins in – fewer than 2.5 million. So that means an absolute maximum of around 2.5 million people own bitcoins right now."

Even if this number is correct, and I think it is, I would like to make a difference between people who are active and passive. Probably from this 2.5 million people just 10 % - 25 % are active people, who use btc everyday. And make at least one transaction everyday.

I showed btc to my sister, she one one couple free poker tournaments on nitrogen. I showed to her how to transfer it in her wallet and it stay like that for months.

i was involved in bitcoins for a long time before i came to this forum so i wouldn't hold it as accountable for most users of bitcoin...