You can't really say that a score is 'good' as it measures attention - which could be good or bad.
Furthermore the average score for journals varies: an article in Science or Nature will typically score much higher than one in a smaller journal, not least because more people have read it and are thus more likely to share it. A good score for one journal might be a low one somewhere else.
Bearing those two things in mind - in general if an article scores 20 or more then it's doing far better than most of its contemporaries.