Very cool!

I've read that LDA doesn't work well on short documents. Your approach of concatenating all tweets for a user appears to work quite well. One other technique I've seen is to concatenate multiple tweets together that contain the same hashtag.

One of our intern students at 99designs did some work on applying LDA to classify graphic design tasks:

http://99designs.com.au/tech-blog/blog/2014/01/22/Swiftly-Ma...

.. you might find it interesting. :)