Not sure, it is nice to get more people onto a game service, but doesn't mean they will spend any money to keep the servers going.
The more users there are, the more likely an ad is to target them.
I'd prefer to have 10000 users browsing an average of 100 pages per month (1,000,000 page views) getting served $0.50-$1.75 ads with a CTR and a CTR ratio of 0.1% (1000 clicks at an average of $1.12 per click) or $1,120 plus the 1,000,000 views as $0.50 CPM (1000) or another $500 for a total of $1,620 (probably on the lower end of $1000 though) over 1000 members doing the same but only 100 of them paying $10 per year.
You have to look at it from a quantity perspective with advertising as the main model.
From the people who do join through referrals, you can sell them an ad-free pack of $2.99 per year to increase revenue drastically from those rough figures, too. You could probably get 100 out of the 10,000 to pay that and increase monthly revenue by $12.
But, with the more referrals, the more odds you have at monetization other than having paid upgrades.
I did on the low end ($1000-1620) because I would strategically place ads and disable the most annoying of them all, the #google_vignette overlay that would have users click their back buttons immediately.
Also, 1 million page views should give you leverage to get 1 sponsor that could pay more ($1500-2000).