[ajiang]

I think this misses the points from the original post, the most critical IMO are:

(1) Recipient generally doesn't know they're being tracked. Even in the argument that 'email is catching up to IM', the recipient always knows that read receipts are turned on -- because it is built into the UI.

(2) Recipient doesn't know they're being tracked multiple times, with multiple locations. Again, in that same argument, messenger doesn't tell you the number of times I viewed your message and where I am when I viewed it.

BTW -- you can absolutely turn off read receipts across both Facebook and Twitter.

[jbob2000]

Turning off read receipts doesn't actually turn off read receipts, it just does't show the state to the user. Facebook still knows whether or not my message was read.