The Valley loves a tale of a breakout success that finds a million users on launch day, but such things are usually the exception. While it’s great to celebrate these wins, it’s equally (or perhaps even more) important to study the companies that launch to moderate success but then twist and tweak their formula until their userbase balloons.

Take Safe Shepherd, a 500 Startups-backed company focused on removing your personal info from the myriad creepy PeopleFinder sites around the net. After watching their userbase climb slowly-but-steadily since launching over 6 months ago, SafeShepherd found a way to more than double their userbase in the past month alone.

From October 1st, 2011 until April 1st of this year, the company had managed to pull in about 7,400 users. They’d helped these users find roughly 200,000 personal records floating around the Internet, and had managed to get around 20,000 of these records expunged.

Somewhere in the past few weeks, however, they found their spark. Their userbase has more than doubled, now sitting at 16,500 users. They’ve found over 140,000 private records in the first three weeks of April alone and have hunted down and destroyed nearly 42,000 of them. To put it another way: In the past month, they’ve hunted down twice as many records as they had in their first six months post-launch all together.

So, what changed?

CEO Rob Leshner chalks it up to an Internet audience that is starting to care more about privacy:

“We launched Safe Shepherd October 2011, knowing that privacy was a dormant issue.  But lately, internet users are waking up to the fact that privacy isn’t dead, and are freaking out about how much information about themselves it out there”

Of course, the general populace doesn’t just collectively get smarter overnight. There has to be something else at play here, right?

And there is! You see, Safe Shepherd launched a referral program back in late February, just weeks before their growth graph spiked. Refer three friends to the service, and you get Safe Shepherd’s premium service (generally $14 per month) for free for life.

Add in a fresh burst of press coverage right at the beginning of April, and everything starts to make sense. They tweaked the service in such a way that would incentivize sharing it amongst your social circles, while perfectly timing the press coverage for an influx of fresh-faced users to help spread the word.

If you build it, they might come — if you build it and give them free stuff for telling their friends, they’ll come in droves.

Now, lets see if they can pull off a much more difficult challenge: doubling down for a second month in a row.