In 2007, I signed up for a social networking site called Doostang. I accidentally allowed it to comb through my Gmail history and invite everyone it found (about 1000 email addresses!). I received about 100 responses to my invitation, some accepting my invite, some rejecting whilst offering comments like "I don't remember us being friends."
This chart shows the result. Most people responded very quickly, either positively or negatively, but the last response came several months after the invitations were sent. And of course, about 900 email addresses did not respond at all.