- riffraff (noun) people who are not respectable : people who have very low social status.
Merriam-Webster doesn't say anything about number. The Free Dictionary says it can sometimes function as a plural noun but doesn't explain when or how. Google books doesn't help much, there are several examples of it both as a singular and a plural noun.
Looking at these two sentences, I can't find any structural difference that might lead one to use a singular or a plural verb. If one is writing a paper on social science and they want to use the word "riffraff", does it matter which verbal number is used?