Iirc word histograms almost uniquely identify authors. Of course this is on larger amounts of text, but I guess you could identify users over seperate platforms this way.
E.g. Intend to use ellipsis (...) to separate thoughts in online conversation a lot. But I try to not do that in reddit, where I try to stay somewhat anonymous.
Still, I assume that it would be possible to correlate my reddit and HN account just by comparing the word histograms (ie which words I use and how often).
(As a complete aside, that program of study also included Forensic Linguistics which truly fascinating. And of course the work of Claude Shannon and information theory, though not in any great depth)