It's not uncommon to require at least 50% of pixels in view for 1 second before you have an impression for static images [0, 1]. AOL defines an impression requiring 2 seconds for images [1]. Facebook likely did some analyses to find that 3 seconds was a good cutoff for their site.
There are more sophisticated ways to estimate dwell time but they seem uncommon in practice; perhaps due to their difficulty communicating to advertisers what the impression metric actually means.
For sites with many bots or inbound marketing you often find users bounce quickly which drives some of this timing. I'm a bit surprised it needs to be that high for Facebook without many bots or users bouncing quickly. Perhaps this is for mobile users scrolling quickly.
[0] http://advertising.aol.com/specs/terms/aol-viewability-terms [1] http://mediaratingcouncil.org/063014%20Viewable%20Ad%20Impre...