"Frog Farming" is a term I heard used by my female cousin and her friends. Basically, it's when you date guys/girls that you find marginally attractive (in any way) in the hopes that they will transform from a "Frog" to a "Prince".
Those who go through this process and consistently fail in this transformation then become "Frog Farmers" as they are cultivating relationships that are destined to yield nothing more than Frogs, not Princes.
The alternative to this, is to only date those that you think are already at a "Prince" status, but those are few and far between, thus yielding many lonely times.
Hope this clears it up.