In the case of Facebook, the company says data may hang around until the URL in question is reused, which is usually "after a short period of time", though obviously that time can vary considerably.
The African delegation came away impressed that Indian small farmers had guided and influenced policy from the bottom up, and it opened their eyes to the enormous possibilities in their own countries.
Blind people, without even being aware of the fact, are actually using echoes of their own footsteps and of other sounds, to sense the presence of obstacles.