How does a social community like Instagram start from 0 to 1K to 10K to 100K users?

What are some strategies did apps like Instagram and SnapChat use when they were completely empty upon launch? How did they grow their initial communities?

The first 1K users for any platform is usually people in your network -- family, friends, and their friends.

It's your job as CEO to get those initial 1K users BEFORE launch.

Before I list out the common ways to get traction, do understand that after the hype of the launch runs out, you're left with the grim reality that -- it takes time and perseverance.

Did you know: Pinterest was only able to get 10K the first year in business?

White hat ways to get traction:

• Social: Instagram photos got shared a lot on Facebook and Twitter.
• Search
• Piggyback: AirBnB got a lot of press from piggybacking off the 2008 Democratic National Convention in Denver.
• Hyperlocal: Facebook was initially just for Harvard university.
• Word of mouth
• Viral components: Dropbox with their "invite a friend to get 250mb free".
• Existing audience: If you already own a popular community / audience, you can make them aware of your new app. Stackoverflow got it's traction from Joel Spolsky's and Jeff Atwood's blog.
• Cooperation: Brighter teamed up with Careington (a network of dentists) to get it's traction.

Black hat ways to get traction:

• AirBnB gained most of it's initial traction by spamming Craigslist. They apologized when discovered afterwards (and stopped), but there is no doubt it was instrumental for their initial success.
• PlentyofFish got all it's traction from creating a gazillion fake profiles and having them show in Google for long tail keywords (people's names initially).
  
• Hire a cloning company to clone Matt Cutts. Have the clone go to work at Google as normal (all he needs is an orange polo shirt and he won't get detected) and give your domain a manual boost, and penalize all your competitors.