After an upgrade to Meteor 1.2 of a year-old project (v1.0.4), the only thing that somehow broke is the integration with accounts-facebook and I cannot use Accounts.loginWithFacebook anymore. The package is installed, and I already tried to remove it and add it again to see if it was a dependency issue.
Thank you @floydprice and @sergiotapia. As you mentioned, I was calling Accounts.loginWithFacebook when I should have been using Meteor.loginWithFacebook… thank you so much and sorry for this quite embarrassing mistake.