The Fitbit Blaze is enjoying strong early demand, Image source: Fitbit,

What: Shares of Fitbit (NYSE:FIT) rose 13.1% Thursday after the health-centric technology company revealed it has sold more than one million Fitbit Blaze and 1 million Fitbit Alta devices in their first month of availability. 

So what: The former, in particular, may come as a surprise to many industry watchers, especially considering this makes Fitbit Blaze the No. 1 best-selling device in both Amazon's smart watch and heart monitor categories. Also recall Fitbit stock got clobbered in January after Fitbit introduced Blaze at this year's Consumer Electronics Show, partly as underwhelmed investors feared its failure to allow third-party apps would prove its undoing.

Now what: In retrospect, however, this isn't entirely shocking. After all, the devices have garnered largely positive reviews from both users and critics alike. And when Fitbit announced decent fourth-quarter earnings last month, the stock plunged after the company blamed soft earnings guidance on plans to invest heavily in sales and marketing related to both Blaze and Alta.

At the very least, Fitbit shareholders should be encouraged those marketing investments appear to be yielding fruit -- even if it means the company is sacrificing profits in the near term. So while I'm personally content continuing to watch Fitbit shares from the sidelines until we see more evidence this early demand is sustainable, I can't blame the market for bidding shares up today.