"About 60 percent of adults (18+) Americans have a Facebook account. But we're talking San Francisco here--the Valhalla of Nerdery and all things internet--so lets bump that up to 80 percent (this 30 percent increase is justified by the fact that SF is 30 percent geekier than anywhere else...).
For purposes of the question, let's limit this explicitly to the city of San Francisco--not the entire Bay Area. At night, SF is about 800,000 people strong. Let's assume a little over 20 percent of that population is either too old or young to use Facebook (it would be higher if mothers hadn't taken off FB...). Let's say that leaves 600,000 people (note: working with round numbers is better for these sorts of things). But then we have to take into account the fact people commute to and from SF! Nerds pour out into the South Bay, suits (e.g., bankers) come in from the East. I suspect the city population swells by 50 percent. If we toss in tourists and conference goers and everything else, 1 million people are in San Francisco at 2:30pm on a Friday. Eighty percent of those people use Facebook, so we have 800,000 possible Facebook users at that time.
The final step is to figure out what percent of the 800,000 are using Facebook at 2:30pm. It's later in the day so I suspect it will be higher than at 11:00am. But I don't really have a good baseline to judge. When I walk around my office, roughly 10 to 20 percent of screens I see as around the office are on some non-work related thing at any time. Let's call that 15 percent and assume that all non-work related surfing by FB users includes some sort of use of FB. That means 120,000 people are using FB at 2:30pm on Friday in San Francisco."