The Sonic was the second best selling sub-compact last month in the US, and for the year so far.
The Sonic may be doing OK in the US, but it's not lighting up the Canadian sales charts:
http://www.goodcarbadcar.net/2012/08/canada-july-2012-car-sales-figures.html844 sold in july, 5834 for 2012
Comparable to the Fiat 500 and Kia Soul, and well above the Fit and Mazda2.
But a little ways behind the Yaris for the year (~6800), quite a ways behind the Fiesta (8470) Rio (8038) and Versa (7916); and absolutely killed by the Accent (> 15,000).
This is an especially price-conscious market, and the ambitious MSRP and lack of significant discounts are probably hurting Sonic sales relative to other subcompacts. I think the Sonic is a great offereing in this class, but GM has absolutely zero positive brand cache in this particular segment (Aveo, anyone?). The Sonic pricing in general (and RS model in particular), as with most GM products, seems to be a stretch until the discounts become available.