Lately I've been seeing comments like this surprisingly often, but my firsthand experience tells me otherwise.
A relative had mail stolen from an outgoing mailbox. She filed a complaint and was contacted months later when they caught the guy. They found her checks (to charity) in his house. She was shocked to hear back so much later. Apparently they were on the case the whole time, since it was affecting a lot of people.
I don't doubt they solve cases of serial theft by ordinary citizens.
They are, however, habitually unwilling to investigate Postal employees in all but the most exceptional cases (and the value of the thefts is not sufficient to be exceptional).
In this case, it was actually an inside job, since some of the mail had been deposited in slots at the post office itself!
Only when they want to investigate your case. Most of the time, postal investigators don't care. Maybe, they care if the package is insured/priority/express mail.