This is a tool that lets you search data, some of which was collected on a false(or dishonest) expectation set by facebook that the data will not be searchable in this manner. That is a violation of trust. I say this as a huge fan and user of facebook's privacy features.
In most cases, I am arguing on the other side telling people how misinformed they are when they complain about a lack of privacy features. But this feature even has me blindsided and I am someone who uses almost all facebook privacy features very carefully(I have posts visible to Public to posts visible to only selected people, for example).
I fully expect a new set of privacy controls from facebook specifically around this feature.