Of all the business and investing news over the past week, the number two trending story for investors on social media was Twitter (NYSE:TWTR) absolutely tanking after releasing fourth-quarter earnings. 

The bottom fell out of the stock to the tune of $8.7 billion in market cap in a single day of trading. And that's despite impressive revenues and earnings overall and on a per user basis. So what gives?

Investors in consumer facing, Internet media companies look to a lot more metrics than simply revenues, margins, and earnings. First among those is user growth, a metric that for Twitter was underwhelming at best and abhorrent at worst. For the first time, Twitter veered sharply off the established growth pattern established by other social media behemoths like Facebook (NASDAQ:FB) and LinkedIn (NYSE:LNKD.DL).

Facebook's Instagram, for example, saw double the user growth as Twitter from the second quarter to the fourth last year. And that's just the tip of the iceberg.

In the video below, Motley Fool contributor Jay Jenkins tells us the story and why this is a major red flag for Twitter investors. He also reveals the two other companies that have out innovated Twitter and are stealing users hand over fist (hint: one's the fastest growing international message service and the other's messages disappear after a short 10 seconds).

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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.