Good question. Let me look around for some Venmo audits or something like that, maybe we can use that as an estimate.
Edit: Alright, this isn't Venmo, but since they are a PP subsidiary I used Paypal's financials instead. This is a very rough way about it but I've found info that we can use.
You can find this data here:
https://investor.paypal-corp.com/financial_history.cfm click the most recent entry (Jan 2017)
on page 1 under "Assets held":
Funds receivable & Customer accounts (averaged for the last 4 quarters) = 13.03 Billion
on page 11:
Current Active Customer accounts = 197 million
13.03b / 197m accounts = $65.98 average account balance Also:
on page 11:
Total Transaction Amount in USD for 2016 = 354.014 billion
Total Volume of Transactions for 2016 = 6.129 billion
354.014b / 6.129b = $57.76 Average transaction sizeSide Note: These calculations are kind of poor when compared to bitcoin, we need liquidity for less volatile trading & the ability to transact/cash out, where as paypal just transfers digits from account a to account b, so the 13.03b metric isn't very fair when compared to bitcoin. I'd say the amount of fiat used to transact bitcoin per year (both in & out) would be the best metric for this. Quite literally, that's all the fiat value in the system. Everything else is thin air (let's see everyone try to sell all at once, the marketcap might be 30 billion+ but there isn't 30 billion worth of fiat to back it up).
Now to Venmo: If this is to be believed, Venmo did 20b in transactions for 2016.
http://fortune.com/2016/10/20/venmo-20-billion/I'd have to guess Venmo has similar numbers when it comes to average account balance & average spent. Using the numbers above, we can roughly surmise that Venmo does around 333 million transactions a year (20b/$60). Without a proper "Active Venmo Customer Accounts" number though we can't really take a stab at how much customer assets they are holding.
Braintree, a subsidiary of Paypal, owns Venmo. I see zero financial statements pertaining to Braintree/Venmo, so I'm almost tempted to say that they are included in the Paypal financial report (or they don't have to report it at all, maybe it's considered a separate private entity).
This writeup is rough, but hopefully I've helped some.
This is a very good demonstration on why Bitcoin's marketcap shouldn't be as big as it is, if it is indeed used as a global currency. Most of us just hold it for the future, so we use it more like gold than a currency. It's a very volatile store of value (oxymoron)
