I am not a Facebook person myself but I see no harm in someone spending their time commuting on a train using Facebook instead of something more worthy, e.g. reading a newspaper.
Behind closed doors people plug in to regular TV programming and spend hours on games consoles. There are only so many hours in the day and people who spend hours on social media are spending less time watching regular TV.
Instead of moral panic we need some better numbers. For instance we have unemployment numbers, we could also do with numbers of how many people lost their lives to Facebook/X-Boxes/YouTube/trawling Wikipedia this month.