Suggestion: Show number of players in ad listing
I would personally dislike this. A number of GMs make use of secret, hidden, or 'standby' characters that look like PCs, but, are in-fact GMNPCs perpetrating ruses for the sake of the mystery/mystique/intrigue/whatever of the scenario.
This would horrifically throw-off the ratio which you are seeking.
Neat idea overall. However, some flaws with regards to another notion: what about player attrition? Nearly every DnD-style game suffers heavily from this (not saying only this type, just more familiar with DnD). 10 join, 4 leave instantly after 3 posts here, 2 more fall-off after about a month. How would the game factor those? etc.. etc... etc...