SAN FRANCISCO — When Mark Zuckerberg introduced an online tool called Facebook Connect in 2008, he hailed it as a kind of digital passport to the rest of the internet. In just a few clicks, users would be able to log in to other apps and sites with their Facebook passwords.

The tool was adopted by thousands of other firms, from mom-and-pop publishing companies to high-profile tech outfits like Airbnb and Uber.

Now those outfits could have been exposed to the consequences of an attack on Facebook’s computer systems. On Friday, the company said the account entry keys of at least 50 million Facebook users had been stolen in the largest hack in the company’s 14-year history.

But the impact could be significantly bigger since those stolen credentials could have been used to gain access to so many other sites. Companies that allow customers to log in with Facebook Connect are scrambling to figure out whether their own user accounts have been compromised.