Finding user similarities within informal data sets
As you accept vague answers:
Sranford NLP tools are strong for this kind of stuff. NER, POS Tagger maybe, Parsers, etc. Now for machine learning itself, I would try looking up at WEKA, it's got a lot of filtering, classifiers and clustering methods, including StringToWordVector filters, that are, in my opinion, fundamental to text classification. Mostly, the tags you should be looking for are Text Categorization, Natural Language Processing and even Sentiment Analysis if you will.