A brief bit of googling shows Yahoo mail has similar numbers of users as Hotmail and Gmail, somewhere between 200-400 millionish, hard numbers aren't often announced.
Oh right, I guess meant "not trendy". I know it's pretty widely-used, but it unfortunately and unfairly tends to elicit the same response as when someone tells you they use AOL for their internet.
So it's popular, though has lost it's top spot.