When 450 million WhatsApp subscribers signed up for the mobile messaging service, they probably never thought their personal data might one day be shared with the world’s biggest social network, Facebook.

So on Thursday, privacy advocates sought to block Facebook’s $19 billion purchase of Whats­App, telling the Federal Trade Commission that the sale would harm WhatsApp users by permitting their data to be sucked into Facebook’s massive advertising business.

The Electronic Privacy Information Center (EPIC) and the Center for Digital Democracy say Facebook has a track record of compromising users’ privacy. The company has repeatedly changed its policies on collecting and sharing user data. When Facebook bought Instagram in 2011, users of the photo-sharing app were told that Facebook could begin to collect their data and the pictures they were sharing on the app.

The practices are deceptive, the groups say, because consumers sign on to social networks with the expectation that privacy policies won’t change. That may be particularly true in the case of WhatsApp, which doesn’t place ads in its app and doesn’t store user data on its servers.

“We don’t know your name, your gender . . . we design our system to be as anonymous as possible,” WhatsApp co-founder Jan Koum wrote in a blog post last month.

The sale to Facebook, EPIC argues, could fundamentally undermine the very qualities that first attracted people to Whats­App.

“WhatsApp users could not reasonably have anticipated that by selecting a pro-privacy messaging service, they would subject their data to Facebook’s data-collection practices,” EPIC attorney Julia Horwitz said.

Both companies defended the merger and said they would continue to operate separately. In an e-mail, Koum said phone numbers, contact lists and other data collected by WhatsApp would not be shared with Facebook.

Facebook added: “As we have said repeatedly, WhatsApp will operate as a separate company and will honor its commitments to privacy and security.”

But privacy advocates want stronger assurances. EPIC’s previous complaints have resulted in FTC actions against Google’s use of data in its social network Buzz and changes in Facebook’s privacy policies.

WhatsApp’s global userbase has grown faster than nearly any other social media or Internet firm. Founded in 2009, it is now bigger than Twitter, Instagram and Skype, and more than 70 percent of its subscribers use the nearly free messaging app multiple times a day.