Better advice: Don't input your email credentials into any site that isn't your email host. Simple.

The fact that legitimate sites ask for this information is really bad for consumers. It makes them think it's business as usual to just give away their passwords to anybody that happens to have a website and a button that says "Import my contacts" or "Tell my friends" or "Send me money" or whatever.

Even if we can trust Facebook with this information, it lends an air of nonchalance to the practice.

The problem is that even you think you can trust facebook. It's only a matter of time until this kind of stuff starts there too. I've already gotten lots of spam from them. People even put my email address into facebook without my permission.

It's ruining the internet.

Note that I said, "even if we can trust Facebook". I consider the "if" a significant word in this context.