Where can I find hard academic data (need citations) that supports the
effectiveness of online
training for social service professionals (as opposed to distance
education with college classes)?

Thank you for your question.  
 
However, I believe that to answer it well, your question will require
more time and effort than the average amount of time and effort
associated with this price. Here is a link to guidelines about pricing
your question, in the pricing guide:
https://answers.google.com/answers/pricing.html

Hi,

Here are two links that you might find useful. Alternatively, you can
look www.redbean.com, www.blackboard.com, ODLAA, look up companies
involved in elearning - they usually have quite a few white papers
written by academics.

The following centra links is just an example of what's out there
(anytime, you want to deposit the $2 for the question...let me know).

A Case Study Approach to Blended Learning
http://www.centra.com/download/whitepapers/CaseStudy_BlendedLearning.pdf

Building a Framework for Enterprise Learning
http://www.centra.com/download/whitepapers/Building_Finn_WP.pdf

PD