Here’s the FAQ of Venmo. It seems they may IN THE FUTURE make money by charging merchants who accept Venmo but not from people who are friends & relatives splitting expenses.
Here’s one person who was concerned about what he views as a security flaw, so delinked his debit cardl
http://www.technobuffalo.com/2015/03/03/why-i-unlinked-venmo-from-my-bank-account/
I’ll ask my kids about it and maybe we’ll think about joining too.
One more article:
http://www.investopedia.com/articles/personal-finance/032415/how-safe-venmo-and-why-it-free.asp