Foursquare has added five million registered users in the last 16 months, bringing its total user base to 20 million, Mashable reported.

The three-year-old startup confirmed the 20 million number through a 4sqDay badge, which is being awarded to anyone who uses the app to check in on Monday (April 16 or “four, squared”).

“Thanks to all 20 million of you for making us part of your lives,” the badge’s message reads.

New user growth has slowed since the last time Foursquare announced a milestone. When the company announced it had reached 15 million users last December, it had tripled its userbase and added 10 million users in about one year.

Foursquare users have, however, checked in 2 billion times in the last two years, according to the 4sqDay badge message. On average, that’s 100 times each — suggesting that while checking in may be the least prevalent smartphone activity, those who do use Foursquare do so frequently.

A December 2011 study from Forrester Research found that just 5% of U.S. adults use any location-based check-in services, suggesting that checking in has a long way to go before reaching the mainstream.

But Foursquare’s user base is large and valuable enough to have attracted partnerships with big brands such as American Express, and its newest features emphasize its value as a resource rather than a check-in service. Its new web interface, for instance, focuses on discovery features, and the web version of its recommendation feature Explore now includes 250,000 restaurant menus.