I believe them when they say they don't share, but are you sure about that? Even advertisers that already have my full name and some data, they wouldn't want to link with what Google and Facebook have?
To be concrete, my local supermarket and Amazon have my full name, and partial purchase information (the supermarket via a loyalty card). Both have ways to contact me with promotions. You don't think they'd like to know what I like on Facebook (if I did that) or what I search on Google that might suggest purchase intent?
I think they'd love that data, the platforms just make more money by only allowing them to target with it.
Why would that version be any better than the one that is generally available?
The only difference that I imagine exists is being able to search from the pool of "everyone" rather than "your friends", but that would make it less confident. In the law enforcement version, there would probably be a way to anchor the search to another individual just to improve accuracy to an acceptable level.
I can't imagine it would be that good? I suspect limiting the search space to your friend list (and probably the part of your friend list that you're ever actually likely to meet) is the only way to keep these results even a little accurate.