The Secrets of the World's Most Successful Mobile Apps From Evernote to Instagram to Snapchat, here's a look at how these apps found their early success.

There's hard work. And then there's luck. But there's also that something else that makes all the difference. And it's that edge that differentiates the most successful apps from the rest of them.

Of course, there's one thing that you cannot ignore, and that is building a product that customers want. Considering that you've built a fantastic product, not just in your own eyes but validated through customer feedback, read on for what made these apps a roaring success.

Instagram: Invite influential people to your beta launch.
Founder Kevin Systrom let some influential technology bloggers and contacts -- like Jack Dorsey of Twitter -- try a test version of the app before its official release. Soon, Dorsey was using it to send photos to his Twitter followers. Word eventually spread. Of course, two articles in TechCrunch (one before launch and one on the day of the launch) helped along with word of mouth.

From 25,000 users in the first 24 hours, Instagram grew to 300,000 by its third week, and then into the tens of millions.

What also made a difference in the initial traction was that Instagram launched right after the release of the iPhone 4, which included a much-improved screen and camera. The company also went against the grain by making it easy to push photos from the app into social networks like Facebook and Twitter, rather than locking the photos in the app. Then, a trick to upload photos super-fast made all the difference to users who wanted the experience in that very moment. Instagram managed to 'hide the slowness' by beginning the upload process as soon as photos are taken and by using a relatively small photo size.

Snapchat: Get your target customers buzzing.
The founders first spread the word about Snapchat to college friends at Stanford University, but the app's popularity didn't really start to take-off until it made its way into the high school ranks to become a popular means of communication for teenagers.

One user best highlights its secret sauce to viral growth: its group messaging functionality. When a user sends a snap to multiple friends, the recipients receive a snap indistinguishable from an individualized message. In effect, mass snaps feel personalized. This is the holy grail of messaging platforms: evoking strong emotion with minimal friction.

Of course, since snaps disappear seconds after they are opened, users feel comfortable sending spontaneous and personal messages that they would not want ingrained into digital histories.

Evernote: Get press attention.
In an interview to doeswhat, Phil Libin mentioned what helped Evernote grow exponentially from Day One to more than a million users was the initial press coverage and a closed beta.

TechCrunch wrote about Evernote's closed beta which ignited the spark for its growth. Within days of the coverage, Evernote received a couple thousand signups for its beta. And the first set of users who loved the product helped spread the word around.

What Evernote also did as a strategy in the first couple of years from launch was to obsess about being in all the app store launches on Day One. With every new device or platform launch, the company had the app ready to be submitted. This helped Evernote become one of the showcase apps for all the devices as they launched.

Clear: Be unique. Be simple.
It all started with building an app that was a major leap forward in app design. With no buttons at all, this app's uniqueness was the simplicity of use in a highly cluttered and complex set of to-do list apps. The kicker for the app was in its design or usability with a no-nonsense interface, dominated by natural interaction instincts, stripping it bare of all complex features (often tagged with to-do apps), but one -- maintaining a list, quickly and in a fun way.

The founders of the Clear app were "clear" in their marketing strategy. They went all out to get tech blog coverage based on demos, previews and teaser videos even before it went to market and sold 350,000 copies within nine days of its launch. They invited select members of the media to beta-test the app and to see if the app caught their fancy.

A video demo uploaded to Vimeo during the MacWorld conference in January 2012 -- where Clear was being shown off for the first time -- has been watched more than 814,000 times so far, also built buzz for the app's launch in February 2012.

Camera+: Be aggressive on social.
The founders of Camera+ chose to go big on the launch of their app and went with the most popular social services that were most relevant to their app spread via word of mouth. They chose Flickr, Facebook, Twitter and email for Camera+ and making the sharing feature better than any other similar feature out on the app store.

The launch promotion included a contest for $10,000+ worth of camera equipment and got the word out to their existing mailing list of about 70,000 subscribers.

One of the other insights they had was not to solely depend on domestic sales. The founders realized that apart from the US, many other countries do not have games dominate the top charts. They drew large crowds from international markets, many places becoming Apple's App of the Week.

Their co-founder posted a video on their blog that demonstrated the main feature that was going into version 1.2. No one had this feature available and this post created a lot of buzz for them and helped sustain the enthusiasm through the months.