It is. You're not breaking into someone's Facebook account. You're not accessing their passwords. You're not doing espionage. It can feel a little uncomfortable to leverage the fact that the vast majority of the 2 Billion people on Facebook don't fully understand how much of their online activity is visible to the people who know how to find it. (You, once you purchase Facebook Deep Dive.) Think about it like this. There's a bunch of stuff the police can't learn about you without a warrant. But if your trash is at the street, and they go through it, they can learn those things.
Having your privacy settings in place is like shredding every confidential document before tossing it in the trash. What's left is food containers, partially eaten meals, magazines, catalogs, and the like. That's really enough though. Imagine a pile of trash consisting of dozens of fast food bags, even more baby food jars, and a recycle bin nearly full of empty beer cans. You've already made judgments about the kind of people live there. Even their fitness as parents - all from easily acquired, non-confidential trash. This may be the only class you'll ever love compared to dumpster diving.