Learn Russian: На старых форумах было мало пользователей.

This is the Prepositional plural. The preposition на requires the Prepositional case to show location. For plural nouns, this case usually ends in -ах or -ях (like форумах), and for plural adjectives, it ends in -ых or -их (like старых).

How do I know whether to use -ах or -ях for plural nouns in this case?

It depends on whether the noun is hard or soft. Because форум ends in a hard consonant, it takes the hard ending -ах. If the noun ended in a soft sign (-ь), -й, or a soft vowel like -я or -е, it would take the soft ending -ях.

Why is the verb было in the neuter singular form if we are talking about plural forums and users?

In Russian, when the subject is a quantity word like мало (few) or много (many), the past tense verb must be neuter singular. You are grammatically saying 'There was a small amount of users'.

What case is пользователей, and why?