Edit: Matt Haughey just told me on Twitter that the quantcast numbers are accurate enough - https://www.quantcast.com/metafilter.com
So that's 6M uniques and 16.5M pageviews per month.
Some napkin math:
Let's say 8 staff + servers cost $100,000/month.
So, they need 10,000 people paying $10/month. That's 0.2% of their monthly uniques.
Assuming 10% of the uniques are registered users and 2% of registered users pay, that seems achievable.
Plus, they can keep the ads for non-paying users and also have higher paid plans for those willing to pay. For example - https://twitter.com/mathowie/status/468474737790693376