Technically everyone is losing marketshare to Windows Phone. The relative size of the pie is set. If Microsoft even gets to 1% marketshare, it has to be taken from everyone else in the smartphone space.
Couple of things to note:
1) iPhone's release on a relatively set calendar. Their marketshare quarter over quarter tends to jump during the holiday and then fade after as people anticipate the next device. So if you are looking at quarterly marketshare, the picture always looks the "worst" for Apple right before everyone knows they are releasing a new phone... so basically right now.
2) iPhone raw annual sales numbers aren't falling. The size of the pie is growing as the dumbphone market evaporates and is filled with the cheapest Android phones China can create, but Apple is still one of the top 2 largest smartphone manufacturer by a mile and tends to trade the top spot with Samsung globally depending on where you are in the release cycle. Give the small number of phone models they create compared to other manufacturers, that's pretty damn impressive.
3) They're making way more money at it than anyone else, both per unit sold and as a gross.