Well, actually every holder can create different bitcoin wallets so it is really hard to trace and in general bitcoin is anonymous. The volume of transactions is our only way to determine and make a good estimate of the actual number of people using bitcoin.
but then again, exchanges with maybe a thousand wallets where they generate hundreds of transactions per day with each wallet, will also not be a good measurement tool. i think the best estimates comes from the number of unique registered users on exchanges. getting an estimate closer than that is not possible.