that's only the case if company intentionally delays application process for green card, which usually takes 1.5 year (unless applicant is form the country that hit the per-country limit, like India). 1.5 year "guaranteed" retention is not that significant to deal with visas/GC. Companies that do intentionally delay the GC process, that's different story, they can stretch the application for many years and have leverage over H1B holder. But that's not FB.