The company had initially said it was "tens of thousands" of Instagram users. That said, the company says that an internal investigation determined "that these stored passwords were not internally abused or improperly accessed."
Some have noted that this fits Facebook's MO; report the problem, let time pass, then update that the problem was much worse than initially reported.
In March, security expert Brian Krebs of KrebsonSecurity noted:
The Facebook source said the investigation so far indicates between 200 million and 600 million Facebook users may have had their account passwords stored in plain text and searchable by more than 20,000 Facebook employees. The source said Facebook is still trying to determine how many passwords were exposed and for how long, but so far the inquiry has uncovered archives with plain text user passwords dating back to 2012.
My Facebook insider said access logs showed some 2,000 engineers or developers made approximately nine million internal queries for data elements that contained plain text user passwords. -KrebsonSecurity