It will be written as फ़ेस्बुक
It’s important to note that Facebook is a proper noun and there in Hindi language, we can use it as it is–we don’t have to change its meaning. In Hindi, this type of noun is called “Vyaktivachak–व्यक्तिवाचक संज्ञा”.
Recommended books to explore Hindi language more:
Because Facebook is a product or a service’s name and therefore will be used as it is.