having written a fair amount of lua code, there aren't many libraries out there. Their npm equivalent is called 'luarocks'. To give you an idea of how old the libraries are: many of them mention the perl package it was translated from.
That being said, the community is small and there are indeed way too few libraries available in LuaRocks (slightly over 300). We have been discussing that on the language's mailing list and I will probably talk about it at the upcoming Lua Workshop (http://www.lua.org/wshop13.html).