Last week, when Facebook announced that it was doing better than analysts had expected, its stock wasn’t the only one to rise. Twitter’s share price was up seven per cent the day after Facebook’s report. The stock market works like that, of course. When a company does well, investors assume that similar companies must be benefiting from the same mysterious combination of factors—and that other investors, once they find out, will react the way they did with the first company.

In this case, they were partly right. Like Facebook, Twitter beat analysts’ financial forecasts. It brought in two hundred and forty-three million dollars in the fourth quarter, compared with analysts’ predictions of two hundred and eighteen million dollars. It turned a profit, by one measure, when investors had expected a loss. Twitter achieved this by extracting a lot of revenue from each user, even as its number of users grew only a little from the previous quarter—exactly as Facebook had. But instead of sending Twitter’s stock price up, like they had with Facebook, investors pushed it down eleven per cent in after-hours trading.

This seemed odd, at first, even to the most seasoned followers of the companies: If Twitter had done so well financially—just as Facebook had—why was its stock price falling? Investors seemed focussed on the fact that the number of people logging into Twitter at least once a month had grown only a little. On average, Twitter had two hundred and forty-one million monthly users in the fourth quarter of 2013—a meagre increase of four per cent from the previous quarter. For comparison, Facebook had 1.2 billion monthly users in the same quarter, up only three per cent from the previous quarter. The growth rate was similar, and yet investors had completely different reactions. There was a simple reason: they don’t actually think of Facebook and Twitter in the same way at all.

Facebook is, to put it plainly, a huge, established company. Its monthly users are equal to seventeen per cent of the world’s population (especially impressive when you consider that only thirty-nine per cent of the population was using the Internet in 2013). Twitter’s monthly users represent only three per cent of the world’s population—which means, theoretically, that it has a lot more room for user growth than Facebook does.

Investors aren’t the only ones who see Facebook and Twitter differently. When Facebook launched in 2004 at Harvard University, it was immediately a hit. Maria Konnikova wrote on Tuesday that when she joined, fifteen days after Facebook’s launch, more than five thousand others had already beat her—and that was when Facebook was restricted to people with Harvard e-mail addresses. When Twitter was founded in 2006, on the other hand, people didn’t really know what to do with it: What was the point, for a civilian, of sharing hundred-and-forty-character messages with the public? The launch was “a whimper,” the Twitter co-founder Evan Williams once wrote. The site gained traction later, at least partly thanks to canny marketing and public relations—most notably, paid displays at the 2007 South by Southwest conference. All along, an outsized proportion of Twitter users have been celebrities and media figures, while a lot of regular people—the kind who flocked to Facebook—remain puzzled by it.

Dick Costolo, Twitter’s C.E.O., acknowledged in a call with analysts on Wednesday that Twitter is still difficult for regular people to understand. “We will continue to make the product easier to use,” he said. To that end, he said Twitter would make some changes to how it works over the next year—making it easier, for example, for people to find existing friends on the site and have one-on-one conversations. Costolo said Twitter hoped to change the “slope” of its rate of user growth. But, tellingly, he wasn’t much more specific than that. “What exactly the slope of that curve looks like and when it will occur is not something that I can guess at,” he admitted.

This didn’t inspire much confidence, it seems. As the call wound down, in fact, Twitter’s share price tumbled even more in after-hours trading. By six o’clock, it was down eighteen per cent. Facebook, meanwhile, had slipped only two per cent.

Illustration by Jordan Awan.