I don't think that's true... LinkedIn was a founded in 2002, while Twitter was founded in 2006. LinkedIn also has different base features and goals of the site that are very different from Twitter. Not to mention that they function very differently in the end, aside from the basics of all social media of posting, replying, liking, messaging, etc. It wouldn't make any sense if LinkedIn's systems were built from Twitter code base, especially since Twitter wouldn't have a code base until at least 4 years after LinkedIn started. As a programmer myself, it would probably be a bigger hassle to take an original code base to build something different vs building that something different from scratch.