The delay in reporting her missing could have been related to several factors. Maybe her friends wrongly thought they would have to wait 24hours to report her missing, I believe this is no longer the case.
It could also be that she had plans for the following day that she told her friend about, the one she was visiting, so maybe for her to go off radar for the following day may have been expected and they only really became concerned when she still wasn't back by that afternoon/evening.
I know from personal experience when a friend of mine went missing by the time we realised, we waited for a good few hours before contacting the police and that was with very genuine reason, the same could apply to this situation although the circumstances were very very different.
I agree, people have busy lives. I'm notorious for not messaging my friends back straight away ...very often its a few days later. If Sarah lived alone it would also add to the time in which it was noticed that Sarah was missing.