[UUMMUU]

Dunno udemy.

Coursera seems to offer more of an academic lesson plan (learn linear algebra, algorithms, history of modern China) which doesn't seem very applicable to a specific tech job. These are more for people who want to learn for the joy of learning. (Same thing would be said about edX although I think it's owned by the schools (could be wrong))

I would say they didn't go after Udacity most likely because of the small, and also younger, user base where as Lynda.com seems to appeal to more of the people who want to transition out of a job into another job.

This is all pure speculation though as I've only done a few courses on udacity and coursera and actually never done any on Lynda.com but have known people who have.