Like most of the trade journals - Library Journal, School Library Journal, PW etc display ads in these are very costly and I believe you only see results if you consistently advertise, every issue, which for most smaller publishers is prohibitive. The big magazines have so many advertisers that I don't think an one ad gets noticed. These ads are much cheaper, there is less competition for space and while their readership is lower, they are serious librarians looking for overlooked books. We always use reviews from the other magazines as the basis for the ads. As always the best way to advertise is to get a good review.
Librarians read their magazines religiously and a good review is taken as a recomendation.
Librarians really do appreciate good books and make the effort to find them. If you attend a library show and show-case your titles librarians appreciate your attendance and take note of books. Surprisingly many people who make the effort of attending a tradeshow never follow up on the contacts made. Develop a good mailing list and keep in touch with your contacts. Find out who the selectors for libraries are - many libraries have a central selection office and require review copies.