The common courtesy of waiting roughly 5 minutes for a friend to respond after commenting on or "liking" a status on Facebook. If the friend doesn't respond within this grace period, one can assume they're offline or simply disinterested in your post.
Mom: "Katrina, it's 3:30am! Get off the damn computer and get some sleep."
Katrina: "Give me a second, I'm waiting for Jessica to write on my wall. I "liked" her status 4 minutes ago. I gotta wait for the 5-minute Rule."
Mom: "You don't have any friends...so stop pretending! And switch the damn laundry before you go to bed."