More than 7 million users are using Instagram daily, more than the number of Twitter users. Impressive, eh?

In the United States during August, 7.3 million users actively used Instagram, while 6.9 million used Twitter.

The numbers mean a feat for Instagram, but it does not mean the social mobile users are tired of Twitter. In fact, Twitter's figures are up 24% over the same six month period, according to comScore, an online tracking and analytics firm.

In addition, Twitter got more unique visitors, with 29 million unique checks to the newsfeed. Instagram lags behind by 7 million, with 22 million unique feed visits.

So both Instagram and Twitter are doing pretty well. But why is the former, only two years old going mobile, ahead in terms of active users now?

Here are 3 of the reasons why Instagram is growing in popularity:

1. The Facebook Factor

Facebook agreed to buy Instagram in April, and the U.S. Federal Trade Commission completed the process in August.

"I'd say Instagram has really benefitted by being bought by Facebook ," Zeus Kerravala, an analyst with ZK Research, told Computer World.

2. The "Visual Element"

It is perhaps more fun for more users to see funny and creative photos than to decipher a 140-character update.

"Well, they're focused on photos and the visual element, which is very popular these days," said Mr. Kerravala.

3. A thousand words all at once

For the photo uploader, a picture does paint a thousand words. A photo can hardly be described in a couple of words. Thinking of a concise way to express concepts and ideas is also not easy for many people.

"I think Instagram is easier to use for most people. It's visually centric," Mr. Kerravala added.

Where do users stay longer?

Instagram visitors spent an average of 257 minutes browsing and uploading photos using their mobile phones. In contrast, Twitter users spent an average of 170 minutes checking the micro updates, Daily Mail reported. This can be attributed to two factors: uploading time and visual contact.

Reading and posting 140-character texts happen fast. Uploading an Instagram photo is relatively easy. But looking at a really good photo can take a couple more seconds. Whatever makes people stay longer at Instagram, it can be assumed Facebook is watching. And soon, Facebook will take advantage of this and leverage the Instagram element in the industry.