I was on FB in 2005, and it was definitely not 15-30 mins/day. People were on there for hours, or rather they'd go on, scroll for 15 mins, then come back next hour for another 15 mins, and so on. But again, that sort of behavior was a product of the times, and I really doubt you would get that sort of engagement if you put the same product in front of users today.
But try it! If you're familiar with a database-backed web framework like Django or Rails, it'd probably take a weekend to re-implement the original Facebook. Do it and see if you get any traction.