facebook or Facebook, twitter or Twitter?

The branding for facebook, twitter, delicious and other social media sites spell them with a lower case first letter. Then why are they written so many times with the first letter capitalized?

Of course it should be capitalized when it is the first word in a sentence like facebook has on its sign up page. The name of the company is Facebook, Inc. so maybe writers are referencing to the company rather than the Web page.

If you check out their copyrights, both facebook and twitter use a capital letter. They are also proper nouns so technically they should be capitalized. But doesn't their branding over ride that grammar rule?

The YAHOO! branding uses capital letters with an exclamation mark, but we rarely see it written that way either.

Even Wired spells facebook with a capital f when they mention it in an blog, but they use a lower case letter when they list it as a tag at the end of the page.

To be grammatically correct, do you spell them with a capital letter or lower case? What is the prudent proof reader to do?