Seems to be quite the subject these days.
My take. It does not matter. Unless your startup has over 50% of the U.S. using it, has raised hundreds of millions of dollars in venture capital, went public at a $100 billion valuation, shrewdly maximized the money put in its coffers by pushing its IPO price to the limit, and you have somehow managed to remain CEO while still controling the company. Then it matters. That has happened once in the history of the world. Facebook, like its purchase of Instagram, is an anomaly. It does not matter.
Sure early stage valuations may come down. They should. What Facebook has shown is that private valuations are out of line with public valuations.
That does not mean startups can not raise good money or find good exits. While the argument can be made that the Facebook impact had not worked its way through the system yet, Oracle's purchase of Vitrue for a reported $300 million and Salesforce's purchase of Buddy Media for $689 million are exhibit A and B. Both companies in the social space. Both companies with solid exits for all involved. Both companies valued at north of 10x revenues. Everybody is happy.
For the vast majority of startups Facebook's impact on valuations does not matter. It is an externality that may not impact you and even if it does it is one that you can not control. Go build your business with a focus on getting to profitbility with as little investment as possible.
You do that and everything else will take care of itself.