Importantly however, Venmo doesn't make money on peer to peer transactions that use bank accounts, which I'd imagine is what most people do as that's free.
I don’t see how that data is all that valuable. There’s the inherent network effect of needing to install it to send/receive money from a friend who did.
But a surprising number of people will pay for instant transfer(almost all of that 300m probably comes from this), and the debit cards they get to take in a portion of the merchant transaction fees. They’ll probably be profitable off that, with p2p as a loss leader to drive installs
Furthermore, just because you can link people together, it doesn't mean that knowledge is actually valuable. The only social network you can link to Venmo currently is Facebook. The only other way to "discover" contacts/friends is by uploading your contact book to Venmo's servers.