FB bug #8178
Currently, searching for profiles by email address may bring up people who have
not actually subscribe to Facebook, and adding them as friends would result in
an invitation to join Facebook being sent.
I’m not sure how these people end up in the search result in the first place.
Perhaps FB culls their addresses from somewhere (if so, this would be
unethical), perhaps they have been invited by someone else; if so, it’s still a
privacy violation as it confirms the existence of that account.
This also causes a misperception that the people involved endorse Facebook (or
trust it enough to own a profile there, at any rate), which sounds legally iffy
— and, on the flip side, if it results in said people receiving more FB
invites than they would otherwise get (if they are not listed), potentially a
violation of anti-spamming laws.
This could be easily rectified — mark such profiles as either “Invited, no
response” or “Invited, declined”, and in the latter case, disallow further
invitations from being sent. Nobody should be nagged to join a service more
than once, if the service provider could help it!