Twitter (TWTR) reports earnings Wednesday after market close. Closing its first quarter as a publicly traded company, investors will be watching closely. While a number of metrics are worth looking into when the results are released, one metric is particularly important for long-term investors: monthly active users, or MAUs. With less than a fifth of Facebook's MAUs, investors should hope Twitter's MAUs are still growing robustly in order to leverage its profit potential.

Image source: Twitter.

Value matters
If Twitter was still trading at its $26 IPO price, eyeing Twitter's growth trajectory wouldn't be as important. But with shares up 140% from those levels, the stakes have been raised, making growth incredibly important to stock's story.

While revenue growth is obviously important, Twitter is ultimately dependent on its MAUs which are the foundation of a social network. Not only do they drive advertiser interest and other revenue opportunities, but they are also the foundation of a social network's competitive advantage -- a network effect. The more MAUs a company has relative to its peers, the larger and more effective the network effect.

Given Twitter's extremely forward-looking valuation, investors should hope that it maintains current growth levels in the fourth quarter. In Twitter's third quarter, total MAUs were up 39% year over year and 6.4% sequentially. For Twitter to grow its MAUs at about 6% sequentially in its fourth quarter, it would need to report about 246 million MAUs, or a 34% gain from the year-ago quarter.

Is Twitter's slowing U.S. growth a concern?
But investors should look at Twitter's regional MAU growth, too. Its U.S. market is especially important.

While Twitter's U.S. market is still growing nicely, the growth is decelerating. Twitter's average sequential growth rate in U.S. MAUs in the past four quarters was 7.4%. While that growth rate handily beats Facebook's 1% average rate in its U.S. and Canada market, it's notably slower than the Twitter's U.S. average a year ago of 10.3%.

For Twitter's total MAU base to keep up its current growth, either U.S. or international growth must accelerate. But the former would be preferable to the latter; the value of Twitter's international growth doesn't come close to the value of its domestic growth. In the U.S., Twitter recorded $2.58 in ad revenue per 1,000 timeline views, or ARPTV, in the previous quarter, compared to just $0.36 in ARPTV internationally.

Can Twitter live up to its valuation?
That's the big question going into Twitter's fourth-quarter earnings report. Trading at 66 times sales compared to Facebook at 24 times sales, Twitter will need to post some big numbers to keep investors happy. So keep an eye on Twitter's MAU growth -- particularly its U.S. MAU growth.