This is not always accidental. Facebook, for example, does not allow a '+' in your email address and you cannot tell me that the engineers at Facebook don't kno

This is not always accidental. Facebook, for example, does not allow a '+' in your email address and you cannot tell me that the engineers at Facebook don't know about the "+" in gmail addresses. The simplest explanation is that Facebook wants to make sure you are not signing up with an alias account. That is, they want your _real_ address, just like they insist on having your real name and other personal info.