Basically Limewire has two modes: Ultrapeer and Leaf

People with fast connections will sometimes automatically become an Ultrapeer. They handle a large amount of traffic and act as hubs at the centre of a number of leafs. The shielding bit refers to this - that since you are a leaf the ultrapeer handles most of the searches in your part of Gnutella so you don't see them. This does not restrict your ability to search, however.

The statistics for available files mean nothing as some clients don't handle them properly (or at all).

Most serious P2P users have more than one client to increase the possibility of finding files. Sometimes files that don't appear in one will appear in another (especially if you have a spread of different networks). On OSX you could try the following apart from Limewire:

Phex (info on this forum) - GNUTELLA
Acquisition (Limewire clone with native GUI) - GNUTELLA
iSwipe - MULTIPLE NETS
eDonkey2000 - EDONKEY NET
CarrachoX - CARRACHO NET

You'll find links for them all on any search engine like Google.