This (or a variant of it) is a widely cited fact in the literature, often referring to people throughout the livestock value chain (not just primary agricultural producers). However, having tracked back through various levels of citations, including several blind alleys, as far as I can tell the 1.3 billion originated as a misrepresentation of a passage in Thornton et al., 2009, (http://dx.doi.org/10.1016/j.agsy.2009.05.002) that states, "Of the planet’s 1.3 billion poor people, at least 90% of them are located in Asia and sub-Saharan Africa, and climate change will have major impacts on the more than 600 million people who depend on livestock for their livelihoods".
The 600 million (also common) appears to have derived from Thornton et al.'s, 2002, (http://books.google.co.uk/books?id=4qAikQ8kOxoC) Table 7 which has an illustrative number of poor livestock keepers (in 2000) of 555.8 million.
The equally common "There are about 500 million small farms in developing countries, supporting almost 2 billion people" is also of dubious origin.
Hopefully the completion of the 2010 round of World Agricultural Censuses (http://www.fao.org/economic/ess/ess-wca/en/) will help us all to collectively refresh our understandings in these areas.
Policy Research Adviser