I think Apple's iMessage did this the best: read receipts are off by default, and "delivered" really just means it made it to the device, no more, no less. The new "Seen" aspect for Facebook messages is particularly sneaky, and I'm not a fan, but whatever. I expect people to respond in time eventually.
Since Facebook launched the "seen" feature, my friends have used Facebook messages significantly more often. Today's communication landscape is filled with tools that leave you uncertain if the recipients have seen or will ever see your message. Facebook messages will see more adoption because they remove that uncertainty.