You can use ARPU to compare one business with another, and some financial reporting also allows you to look at profit per user. ARPU offers one-half of the equation needed to know how much revenue a business has, the other half being the total number of users.
Both numbers are important. However, depending on the industry, ARPU is arguably more important than the number of users. The social media industry typically makes money from advertising. So, it's easier to grow revenue by increasing ARPU than by adding new users.