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Facebook does consume roughly one and a half percent of Americans' waking hours. ( highly rough estimate based on http://mashable.com/2011/09/30/wasting-time-on-facebook/ )

These various things aren't directly commensurate, but it shows a certain level of relevance to Americans. (And, though I shudder to admit it, there are other countries in the world.)

Check your units: GDP is $/year; your Facebook is $; $/($/year) = year. You should end up with a result in years, not in percent (which is unitless)

Right, the Facebook number should be change in worth per year for a proper comparison. Which is lower taken over the entire company's lifetime, but higher if you just include the last year.

"The USA produces 200 facebook-equivalents a year" doesn't have the same impact, I guess.

If the $75 billion was revenue rather than worth, sure. Otherwise you're comparing a static number (FB's valuation) to a per year "flow" (GDP).