But if you have a network telling what they know to whomever will listen, you also have those in the network telling what they suppose, or guess, or even! what they know to be false, but what makes too good a story to NOT tell.

That is why Sir John knew of Mrs. Dashwood's downfall, I suppose (not servants, necessarily, but SOMEONE talking of the financial ills of someone else), since he had not visited the Dashwoods since before their move to Norland, eleven and a half years ago, and lived quite a distance away. And that may be how information on Willoughby's estate, and expected inheritance, became common knowledge (although Willoughby himself is not reticent, and may have told the true state of his affairs, himself).