Most people like the technology/startup scene and the culture of the city (for me, it's only the former). But even if you ignore that, the relevant financial statistic is your disposable income, not the percentage of your pay that goes to rent.

At $70k a year, you should take home roughly $70k / 12 * 0.65 = $3.8k each month.

At $120k a year (rough estimate for a Facebook engineer, so correct me if this part is unrealistic), you should take home roughly $120k / 12 * 0.65 = $6.5k each month. If you're paying $3k each month on rent (which is quite generous if you're living by yourself), that leaves you with $3.5k each month.

That means that your $70k job only leaves you $300 a month for rent, in order to have the same amount of money left over. That's really pushing it, considering it's only half of the median USA apartment rental, and considering I paid $400 a month for a modest apartment 4 years ago in Springfield, MO, which as I understand it is in one of the cheapest areas in the country.

Obviously there are other effects here that I'm missing. My 65% take home portion could be completely wrong. Food prices are almost surely higher in SF (of course, most startups and tech companies provide some free meals).