PayPal (NASDAQ: PYPL) has been very slow to monetize Venmo, its mobile-first peer-to-peer payments app. It's controlled the company for five years, and it's grown payments volume on the app to $46.4 billion on a trailing-12-month basis. But PayPal's revenue from that mountain of payments is still minuscule compared to its more mature flagship product.Interestingly, PayPal's original product started off with a similar business as Venmo (hence the name).