To bad that you are using FileMaker 12, a new summary field in FileMaker 13 would give you yet another option that is quite simple for getting this count.
With 12, your options for this count are to use either
ExecuteSQL: A new way to count unique values in FileMaker 12
Or a rather bizarre, long standing method called "Sum the reciprocal": How to count the number of unique occurences in field.
For those curious about how this might be easier in FileMaker 13, there's a new "list of" summary field that would produce a return separated list of all the DRF numbers in your found set. A calculation field can copy that value so that the calculation field can be used as a match field in a relationship to a new occurrence of DRF records. A count function or summary field that then counts the related DRF Records via this new occurrence would return your count. All without using SQL or Sum the reciprocal.
The file does not seem to really be available using the 4Shared service. In addition, beginning to use the Facebook method also linked there reveals that it will download your Facebook contact list. Really? You expect us to do that just to download an example file??