Look at the title of post - i'm not showing you HTTP is plaintext based - I said exactly "Never use the same login for 2 websites"
I call BS to that. Fair enough for websites that use your banking or credit card information, but for the rest? I don't think so.
For irrelevant sites such as Hacker News, Reddit, whatever other minor web 2 site you can think of, you should ALWAYS use the same password. Why fart around with a ridiculous number of passwords for websites that are nothing more than minor daily distractions?
You're also ignoring reputation. Breaking your Facebook account may not be a huge win by itself, but if I can use it to get your 50 Facebook friends to load a Javascript worm, I might have probably 40-50% of their bank passwords. Look what happened with Orkut a few months back.