The link should really be to the original report [0]. This 55% is iPhone's share of phone shipments in Q2, not the number of users of the respective phones at any given time. In other words, fewer people are buying smartphones than were before, but Apple saw less of a hit to their numbers than everyone else did. Contrary to existing comments here, this stat doesn't appear to indicate that people are switching from An…
That's generally what a "market share" number measures: the share of sales during a period of times.
The number of users of devices would be the "installed base."