It's not literally part of the definition of "whistleblower", but it's evidence that you should be really suspicious. If your whistleblower has too much backing, odds are that there's actually someone to gain from the whistleblowing who is doing things from the shadows that you might not be too fond of if they were out in the open.

If the whistleblower has too much backing, odds are also that the company is not particularly bad and the company's being targeted for other reasons. It's a type of shakedown: everyone in the Mafia is a criminal, but when one Mafia family becomes disfavored, the police magically get a full report of their crimes, crack down on it "because we need to get hard on crime", and the other families consolidate their power. Yeah, the disfavored family actually committed the crimes that they're suddenly being arrested for, but the overall effect of the crackdown is not a reduction in crime.

The "whistleblowers" reporting on Facebook are doing so because someone with a lot of influence wants them to. Maybe the government, maybe internal pro-censorship Facebook factions, maybe the press, maybe an actual outside rival. But the ultimate goal of whoever it is is more censorship, but of the proper type.