Some networks are more valuable than others. Raw MAU tells you nothing about the LTV of those AUs. What's better for eBay: a shitload of people who never buy anything? Or a smaller network of engaged, repeat sellers and buyers?
And some networks have negative value. 4chan, despite attracting large traffic numbers and being a well-known cultural force on the Internet, has always struggled to attract advertisers.
To maximize financial value of its user network, a parent company must exclude users and content that subtract value. Examples would be lower-LTV users who drive away higher-LTV users via harassment. Or users who post content so objectionable that brands don't want to advertise against it. MAU growth alone tells you nothing about those things.