Over the past few weeks, Twitter's (TWTR) attracted a lot of attention, and not for the right reasons. The stock is down about 55% since the beginning of the year and investors are worried the company can't grow its user base fast enough

At the core of its misfortunes are three numbers: 255 million, 10, and 1.44. They aren't inherently bad numbers, but so far they've disappointed Twitter's investors.

When 255 million isn't enough
Twitter is a young public company, and being a tech stock with no profit, there's a huge expectation from investors that it should be growing its user base by leaps and bounds. Unfortunately, that's not quite happening.

Twitter's monthly active users (MAU) hit 255 million in Q1 2014, up from 204 million year over year. Here's a look at its impressive growth since 2010:

Source: Statista.

Those numbers look pretty solid from this perspective. But if we take a look at growth as a percentage, Twitter only grew its monthly active users by a little more than 5% sequentially and 25% year over year. Is there still growth? Yes. But it's not enough for a company that's fighting to prove itself a viable investment in the social media market.

But they're really engaged users, right?
Here's where that 10 number comes in. That's the percentage of decline Twitter saw for some of its important overseas user engagement markets year over year. This means that users abroad are refreshing their timelines much less than they did a year ago. Things are better in America, with a decline of just 3%, but it's still moving in the wrong direction.

While Twitter's overall timeline views were up 15% year over year, MAU timeline views were down 8% from the prior year.

Every nickel counts
The last number that's recently spooked investors was the drop in advertising revenue from $1.49 per 1,000 timeline views in Q4 2013 to $1.44 in the first quarter of this year. To be completely fair, Twitter saw a 96% year-over-year increase for its revenue per 1,000 views. The company said its fourth quarter tends to have higher advertising numbers, so the sequential drop was somewhat expected.

But investors likely don't see it that way. Though we'll have to look closer at this number in future quarters, revenue per 1,000 views is one of the key metrics investors have in assessing how well Twitter is making money from its services. So the sequential drop didn't reassure anyone that Twitter is moving enough in the right direction, despite the huge increase from a year ago.

Foolish thoughts
These three numbers have been a part of Twitter's downfall over the past couple of weeks, but it's not as if the company isn't taking steps to improve its service. It redesigned web user profiles, pushed videos and photos into more prominence within tweets, and made it easier for users to integrate phone contacts. These are the first of what will likely be many steps in growing users and increasing advertising revenue.

Investors should keep an eye on these three categories -- MAU growth, engagement, and revenue per 1,000 views -- in Q2 results to see if Twitter's new features are paying off. The company is supposed to be a growth stock, and right now it appears investors are doubting that label.