SAN FRANCISCO -- Instagram announced Tuesday that it had hit 100 million monthly active members, a milestone few would have expected when users of the mobile application began quitting in droves after a change in its terms of service was announced late last year.

In a sentimental blog post recalling the company's early days in a drafty office near the San Francisco Giants' home stadium, co-founder and CEO Kevin Systrom said of the milestone, "It's easy to see this as an accomplishment for a company, but I think the truth is that it's an accomplishment for our community.

"Now, more than ever, people are capturing the world in real-time using Instagram -- sharing images from the farthest corners of the globe. What we see as a result is a world more connected and understood through photographs," Systrom added.

Instagram launched as the Giants were pushing toward their first World Series title as a San Francisco franchise in October 2010, and it quickly became a must-have app for smartphone users who wanted to share photographs with their friends. Less than two years later, in April 2012, social-networking giant Facebook agreed to pay $1 billion for the company, though the price later dropped because part of the acquisition was paid in declining Facebook stock.

Instagram's popularity suffered in December, however, when the company announced new terms of service that included a rule that upset users: "A business or other entity may pay us to display your username, likeness, photos (along with any associated metadata), and/or actions you take, in connection with paid or sponsored content or promotions, without any compensation to you."

Users openly revolted, proclaiming on Twitter and other social networks that they were leaving the service behind. Systrom moved quickly, offering a blog post that promised Instagram was not trying to claim ownership of users' photos, sell them, or other nefarious acts Twitter users claimed the company was attempting. Eventually, Instagram reverted to an earlier version of its terms of service, avoiding the controversial language and rules.

Still, it has been reported that the episode cost Instagram part of its growing user base, with The New York Post reporting just before the year ended that Instagram had lost a full quarter of its users because of the controversy.

Instagram disputed the reports -- a spokeswoman said at the time, "We continue to see strong and steady growth in both registered and active users of Instagram" -- but never gave a full accounting of its users until Tuesday.

Still, the young app has a long way to go to reach the user totals of its social-networking brethren. Facebook has more than a billion users worldwide, both Twitter and LinkedIn claim more than 200 million monthly active users, and even Google (GOOG)+ -- which launched a few months after Instagram -- has more than 135 million users active on the service, according to Google executive Vic Gundotra.

Contact Jeremy C. Owens at 408-920-5876; follow him at Twitter.com/mercbizbreak.