1. "Ich danke den Verbrauchern (dative plural)" = I thank the users.
2. "Ich danke dem Verbraucher (dative singular)" = I thank the user.
3. It's possible, however, that they also accepted a plural translation for "Ich danke dem Verbraucher " because in some contexts, the singular can be used to denote a whole group of people or things, and singular and plural are more or less interchangeable. Like in English: "The cow is an animal" = "Cows are animals", "The careful reader will note that..." = "Careful readers will note that...".

@Katherle: While your interpretation is possible, of course, I don't think duolinguo should accept this solution. The pars pro toto construction is a rhetoric technique. Technically, 'the users' is not a correct translation of 'dem Verbraucher'. As we don't have any context in the grammar exercises, I guess accepting this solution does more harm than good. @mauvebean: I'd suggest to report it as an error to duolinguo.