Honestly? I'm willing to chock this up to a mistake. A reasonable series of circumstances for this would also be that they missed a single permissions check on an otherwise private-only method. It's probably a single line of code, and one that exists in thousands of other places, surrounded by at least hundreds of other lines of code. An easy thing to overlook.

That's not how security should work. The default should be no-access, so that missing a line of code or making a small mistake leads to too much restriction rather than not enough. That would also help the developer notice the mistake, since the feature wouldn't work.

Agreed, but you'd be hard-pressed to find any site that has that as the standard (much less a social site, with so many inter-weaving connections), that isn't crammed down their throat by laws. Even then it's still hard to get (and keep) it correct 100% of the time, and stands in the way of making changes and new features, which are what keep social sites alive and competitive.