Users don't pay for something after they've already been receiving it for free.
The overall results are: 1. Not so good publicity, we are no longer treaded as "cool service". 2. Increase in revenue. 3. Significant reduction in traffic and CPU load, hence save on expenses.
I believe the result is that now people consider it more reputable, since for any given group, someone somewhere is willing to spend $18/year for it. On the other hand, they probably lost many users.
I would suggest keeping the core of what you have now free, and reserving some of the bells and whistles for paid accounts. Then to lure the free users (who are not completely disgruntled at this point) over to the paid side, add a great new feature or two.