NEW YORK (AP) - It was a Prime quarter for Amazon.

NEW YORK (AP) It was a Prime quarter for Amazon.

Amazon surprised investors on Thursday with a fourth-quarter profit that soundly beat expectations, despite a continued increase in spending and a slight sales miss, partly linked to the strong dollar. Investors drove shares up over 8 percent in aftermarket trading.

The results seem to indicate that areas in which Amazon has been investing in heavily for years, its $99 annual loyalty program and its Amazon Web Services cloud computing offerings for businesses, are finally helping out its bottom line.

CEO Jeff Bezos said Prime membership grew 53 percent during the year. It doesn't give out total figures. Prime memberships help Amazon get people to engage more often with the company's products making them likely to spend more, says Morningstar analyst R.J. Hottovy.