Actually, it is meteoric.

Linux's market share didn't grow by 0.32%, it grew by 0.32 percentage points. It actually grew by 12.5% month-over-month. That huge. It went from 2.57% to 2.89%, which is only an increase of 0.32 percentage points. But that's because the starting value is such a small percentage. But, the number itself grew by about 12.5%:

2.57 * 1.125 = 2.89

If it could keep up this month-by-month growth it would go from 2.57% to over 10% within 1 year. If it could keep it up for 2 years Linux would be nearly half of all Steam users.

On one hand, I don't think that would happen because the people making the switch now are early adopters and more adventurous users, so at some point it's going to slow down. On the other hand, I think adoption will speed up at some point once there's a critical mass of Linux users and Valve and nVidia start putting even more effort into Linux builds.