Twitter Inc (NASDAQ:TWTR) beat expectations on revenues and earnings per share in its first quarterly report, but it missed on the two most important results. Shares went into a downward spiral right after that report, and of course the comparisons with Facebook Inc (NASDAQ:FB) must continue.

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It took Facebook Inc (NASDAQ:FB) about a year to take off as investors waited to see if the social network could prove its business model. But since we just can’t stop comparing Twitter Inc (NYSE:TWTR) to Facebook, shares of Twitter took off right away as investors anticipate a future similar to what Facebook’s business looks like right now.

Twitter’s growth metrics a big problem

Bernstein Research analysts Carlos Kirjner, Peter Paskhaver and Garrett Marks put the problems with Twitter Inc (NYSE:TWTR)’s growth metrics into perspective. They note that when Facebook Inc (NASDAQ:FB) was about the same size as Twitter is now, it was adding about 13 million new monthly active users, but Twitter added only 1 million. Suddenly the two social networks couldn’t look more different, and indeed it may be a comparison like this which finally put the scare into Wall Street regarding Twitter’s valuation.

They also draw another troubling picture. They said that if Twitter Inc (NYSE:TWTR) is able to quadruple the number of new monthly active users it adds each quarter, it would take approximately nine years to hit 200 million domestic users, which is how many Facebook Inc (NASDAQ:FB) has.

Other big problems for Twitter Inc (NYSE:TWTR) include the fact that the company reported a declining growth rate in both domestic and international monthly users and also what looks like a downward trend in the “absolute number” of new monthly active users. The Bernstein team notes that this looks like Twitter might “go ex-growth” in the U.S. within just a few quarters.

Buying Twitter at $30 a share

They analysts maintained their Market Perform rating and $40 per share price target. They said because of what the first earnings report indicated about Twitter Inc (NYSE:TWTR), they would now get in on the company’s stock at around $30 per share. They slightly lowered their user penetration expectations and are now expecting the company to come up just short of 90 million domestic users by 2018 and 390 million international users.

Twitter Inc (NYSE:TWTR) management assured investors that they would be able to speed up user growth by improving the new user experience in a number of ways. They’re working on better integrating rich media content, organizing content, making content discovery easier and using the network in “conversational mode.” The Bernstein team was not dismissive of these comments and said they might work. However, they note that Twitter’s current user growth rate is “so anemic” compared to its penetration rate that they couldn’t assume that the steps management is taking will have a major impact on growth.