Is it acceptable to contribute a poster to a conference without actually attending? I have a poster presentation that I cannot attend but I have prepared a poster to tell my friend to post for me. Is it common to do a poster presentation at a conference/meeting without being there in person? I have been to a quite a number of conference and this seems like a norm.
Without knowing the particular field or conference:

No, this it's not common to have a conference poster without presenter.

It often happens that poster are on display for the whole conference and nobody stand by their side to explain them and this may give you the impression that there is nobody there to present them. However, usually there is one slot called "poster session" where the presenters are supposed to be with their posters and the all other participants are supposed to walk around and get into discussions about the posters.

However, more importantly:

There may be a rule on attendance stated by the conference.

It may happen, for example, that if some poster had no presenter, the paper that belongs to an accepted poster will not be published in the conference proceedings (or removed from the online proceedings).