Even though Facebook is pushing to convince mobile developers that it is a strong channel for growth, many of the top developers are still finding that a high app store ranking or using the phone’s address book is good enough to get users cheaply.

Very rarely do we see an app that successfully rests across multiple mobile platforms and Facebook.

However Beluga, a tiny but fast-growing group messaging startup, is showing that it is possible to mix viral tactics on Android, iOS and Facebook to pick up users. The three-person company is growing by at least 20% week over week among Facebook users, according to AppData.**

Founded by ex-Googlers Ben Davenport, Lucy Zhang and Jon Perlow, Beluga makes it easy to send messages to multiple friends on a mobile phone. It’s part of a wave of group messaging startups like GroupMe and Fast Society that rely on telephony startup Twilio’s APIs. The company has been bootstrapped so far. The trio are tight-lipped about potential revenue models or the future directions in which they might take the product.

Beluga uses several tactics to on-board users including –

Push notifications every time a friend joins: One creative tactic that Beluga uses is a push notification every time a friend joins. When a user downloads an app, there’s a big chance that they will drop off. In fact, 26 percent of users who download mobile apps use them just once, according to a study from mobile analytics startup Localytics. Reminding users that the app exists when new friends join helps activate “sleepers”, or people who have downloaded the app but have forgotten about it.

Beluga populates the friend list from the address book: After seeing the rapid growth of similar apps like Kik Messenger, Zhang, Davenport and Perlow started to look at the address book as a good way of acquiring users. Beluga does not download contacts directly. Instead, it hashes each name and phone number with a cryptographic key that the company can’t read. But if a new user comes on-board whose contact information matches the hash value, Beluga will alert friends already using the app.

Facebook Log-in on the First Screen: Users have the choice of logging in through Facebook right away. A large fraction (but less than half) of them do.

Option to Post to Facebook Wall upon sign-up: Beluga adopted this idea from Jumo, the social causes startup created by Facebook co-founder Chris Hughes. When users sign-up for the app, they have the option of posting it to their Facebook wall. The app used to automatically post on Facebook, but this violated terms of service so Beluga changed it to an option with posting as the default.

Cross-platform strategy with iPhone and Android access from the beginning: Beluga decided to launch on both smartphone platforms at once, boosting its potential reach. Usually, we see developers go iPhone first and then migrate to Android (although there is an increasing number of Android-first firms). Other developers like gaming company Glu have also take a cross-platform approach with fluid gameplay with friends across devices.

Availability to non-iPhone or Android users through SMS and Twilio: If a user has friends who can’t get the app because they don’t have the right smartphone, they won’t be left out. Beluga will send text messages to members of a group that don’t have a native app. This is where Twilio comes in. Unlike GroupMe, which has to use Twilio for every message it ends, Beluga only has to use it a fraction of the time when it can’t reach users through the native apps. That means the company’s costs are much lower.

“When you get an SMS through Twilio and are able to start chatting right away with friends, you can get upsold to the app,” Zhang said.

Additional invites with email or phone number: Users can invite their friends, who get an SMS through Twilio and can start chatting right away, even without downloading the app.

Posting photos to Facebook: Last but not least, users can post photos to Facebook, letting friends know about the app.

**This number underestimates total usage of the app, but indicates its fast-paced growth. Also keep in mind that the smartphone app market is extremely fragmented and small compared to the web, so any number that would be impressive on the Facebook platform needs to be divided by at least 10, if not 100. The highest number of Facebook MAUs we see for third-party mobile apps tend to be in the 3 to 400,000 range.