Learn English – What do you call the person sitting behind the driver on a bike

I'd say the word is nethead - an enthusiastic/obsessive user of the Internet*.

I do recognise the alternative netizen, but for me at least that implies much about a person's attitudes to, and active participation in, the emerging global society embodied in the Internet.

My 84-year-old mother, for example, is online for many hours every day playing "solo" online games. She almost never uses the Net for interactive communication in any form (maybe 3 emails a year), so whilst I would call her a nethead, I would not call her a netizen.

Perhaps you’re thinking of the bystander effect. You could describe someone who doesn’t meet their responsibilities as negligent (failing to act), impotent (unable to act effectively), or oblivious (not even noticing the responsibility to act).