The contextual/behavioral advertising market is driven by user adoption and advertiser demand. As long as you have a solid value proposition with users, there will be no point at which your inventory exceeds demand.
There will always be unknowns in business. The key metric is the measure of how much you know about the market forces. People who consistently fail know less than a majorty of what they should have known, had they been slightly more paranoid, intelligent or humble. I think I know about 80% of how the Others Online market will play out, which is optimal. 10% is that which you just can't know, and 10% you need to just keep floating out there until it confidently becomes part of the 80%.