From Washington Post article: "A month after the FISA court learned of the program in 2011 and ruled it unconstitutional, the NSA revised its collection procedures to segregate the transactions most likely to contain the communications of Americans. In 2012, the agency also purged the domestic communications that it had collected." (Source: http://www.washingtonpost.com/world/national-security/nsa-ga...)

That said, I really don't see how there could be an argument of an effective oversight, if what you are overseeing is so secret that you have no independent information on the subject matter. The secret organization (NSA in this case) is strongly incentivized to not disclose the entire truth or withhold key limitations in order to achieve its' objectives. If there is no way to independently verify that what is being said/committed to is actually functioning, court pretty much has to trust NSA to self-police. Everything here is being done on the NSA's terms and not the Courts or Congress's.