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Meta made its Llama 2 AI model open-source because 'Zuck has balls,' a former top Facebook engineer says
(www.businessinsider.com)
This is a most excellent place for technology news and articles.
It was probably just to get all of the free development work the community has done. There are multiple engines designed around optimizing llama models specifically such as llama.cpp and exllama, and many other projects built around the architecture.
Facebook's research division also has a pretty consistent track record of releasing things to the public rather than letting their research models rot.