Netflix (NASDAQ:NFLX) has delivered some monster quarters recently, crushing both management's guidance and analyst estimates for global member additions. As a result, the stock has soared, rising about 146% over the past year through last Friday. But Netflix's near-flawless run over the past 12 months suffered a setback this week when the company's second-quarter member growth fell 1 million members below management's guidance -- the first time member growth was below guidance since the first quarter of last year. Shares fell about 14% in after-hours trading.

As investors look over the company's second-quarter update and consider the implications of Netflix's worse-than-expected member growth, here are three metrics from the earnings release that every investor should see.

A group of young people watching TV

Image source: Getty Images.

1. 5.15 million global net member additions

Netflix's lower-than-expected net member additions globally are sending shares lower for a reason. Going into the quarter, it was clear that Netflix's growth in streaming members was arguably the most important metric to watch. Not only is the metric a key driver for the company's revenue growth, but it gives investors a good idea of how well the company is retaining and adding members.

Going into the quarter, management had guided for 6.2 million member additions -- a figure that would have been a second-quarter record. But actual net member additions were 5.15 million -- about in line with the second-quarter record 5.2 million members Netflix gained in the year-ago quarter.

Netflix simply said it "over-forecasted" global net additions, and it reminded investors that the guidance it provides is the same as its internal guidance, "meaning in some quarters we will be high, and other quarters low relative to our guidance." In other words, the company doesn't provide a public estimate that is slightly lower than what management actually expects just so it can consistently beat it.

Netflix's global net member additions have come in below guidance in three of the past nine quarters.

2. 650,000 U.S. member additions

Netflix's U.S. member additions were particularly low -- at least compared to management's guidance for the metric. Netflix added 650,000 new members in the U.S. versus guidance for 1.2 million members.

Netflix's 650,000 net member additions marked a significant deceleration compared to the 1.96 million members the company added in its first quarter. But management was quick to point out that through the first half of the year, total U.S. net adds are actually slightly higher than last year.

3. An 11.8% operating margin

Another key area to watch when Netflix provides its quarterly updates is its operating margin. The important metric has seen nice growth recently, rising 232 basis points year over year to 12%. Expansion of Netflix's operating margin provides important evidence of the scalability of Netflix's business model.

Netflix's second-quarter operating margin was 11.8% -- up 720 basis points year over year. The strong growth drove a 262% year-over-year increase in operating income. The 11.8% operating margin is impressive since it's nearly in line with management's guidance for a second-quarter operating margin of 12% despite Netflix's worse-than-expected member growth.

While investors should certainly keep an eye on Netflix's member growth in the coming quarters to ensure it remains healthy, worse-than-expected growth in the metric in a single quarter isn't enough for investors to adjust their thesis on the stock. The company is still experiencing exceptional growth, with streaming revenue rising 43% year over year. Indeed, Netflix is still firing on all cylinders. As management pointed out in its second-quarter update, "Earnings, margins, and revenue were all in-line with forecast and way up from prior year."

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.