Even if the users of snapchat were particularly concerned with the security of this feature, they alleviated that concern in the post by stating that all of the financial security will be handled by Square which is building it's reputation of security in mobile payments.
I'd like to see a venn diagram of Snapchat users, people who actually know who Square are, and people who don't trust snapchat's security. I'm willing to bet that people who wouldn't trust snapchat would also know enough about security to have heard of, and (possibly) trust Square.
Fair, but even without the information of who/what Square is beforehand, you can look them up. I'd be more likely to trust them than Snapchat simply because they've been around and have multiple payment solutions out already. And even if you can't be motivated to do a few quick searches, you could likely make some assumptions that snapchat didn't pick up some random company off the street to handle their finances.
