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These are too hard to say. Ad networks can give you an estimate judging on your stats (countries, current fill rate, etc...). And they'll give you...
You don't need to switch, you can use both, or more. All the ones mentioned in this thread are good. But as you probably know, be careful with...
Eh, don't be, just pick 2-3 different ones, you don't have to only pick one and you probably shouldn't pick only one anyway. Just pick a new one...
