Shares of Roku (ROKU 1.36%) may have slipped after the streaming-TV company reported earnings last week. But investors shouldn't automatically interpret the stock price pullback as a sign that the company is down and out. In many ways, Roku is executing extremely well and growing very rapidly. But there are also some areas of concern investors will want to keep an eye on in the coming quarters.

Here's a close look at Roku's third-quarter results -- and what they mean for shareholders of the growth stock.

Roku Channel displayed on a TV.

Image source: Roku.

1. Platform revenue soared

While Roku's total revenue grew 51% year over year in Q3 to $680 million, this figure was weighed down by a 26% decline in player revenue. Platform revenue, which accounted for 85% of total revenue, grew 82% year over year to $583 million.

Roku said in its third-quarter shareholder letter that momentum with platform revenue reflects "significant contributions from both content distribution and advertising activities."

2. Improving average revenue per user

Roku's average revenue per user surpassed $40 for the first time ever in Q3, highlighting how well the company is monetizing users on its platform. This figure was up 50% year over year.

This key metric highlights how Roku is able to grow so much faster than Netflix (NFLX 1.04%). Since Roku gets to make money from both subscriptions and advertisements, there's a lot more upside potential to its average revenue per user metric than there is for Netflix's. More importantly, the key Roku metric is growing much faster than Netflix's equivalent: average revenue per membership. Netflix's average revenue per membership rose 7% year over year in Q3. 

3. Rapid growth in monetized ad impressions

Highlighting how critical ads are to Roku's growth story, the company said that total monetized video ads on its platform "nearly doubled year-over-year, driven by strong client acquisition and retention."

4. Strong momentum in its advertising platform

Importantly, Roku also said its ad-buying platform, OneView, continued to see robust momentum. Purpose-built for streaming video, OneView's TV streaming impressions more than doubled year over year, management said.

5. Active account growth has slowed

One potential concern from the quarter could be Roku's slowing account growth. The company added 1.3 million active accounts during the period. This is down from the 1.5 million it added last quarter and 2.4 million in Q1. Management blamed "global supply chain disruptions" in the U.S. TV market for the slowdown.

6. Revenue guidance missed expectations

For Roku's fourth quarter, management said it expected revenue to come in between $885 million and $900 million. This was well below analysts' consensus forecast for revenue of $944 million. 

A weaker-than-expected guide from Roku management boils down to the Roku team being "mindful" of global supply chain disruptions and how they can impact consumer confidence, product pricing, and advertising spend. But management was sure to note that "the secular shift to streaming remains intact."

Roku shareholders should be impressed by the company's monetization momentum but they should also keep an eye on active account growth and revenue growth. While a further deceleration in revenue growth should be expected in Q4, investors should look for more normalization -- or at least more moderate deceleration rates -- in 2022.