Good question. The codebook for the variables on which I relied in producing this summary and also in producing the infographic says "Indicates whether the respondent found the first employer within 12 months after completion of the respondent's 2015-16 bachelor's degree through __ INSERT REFERRAL SOURCE__."
The exact question was "How did you hear about your job with [EMPLOYER NAME]?" It offered a multi-select (select all that apply) response option. The multi-select responds to your other good comment that "in a job search" it is often the case that candidates will see a job via multiple "channels" or multiple "postings" or multiple "friends/connections."
Incidentally, the multi-select option is why I didn't do a pie chart. Because the there will be multicollinearity in the responses.
So hopefully that clears up the ambiguity.