Can I also use in when referring to someone who is already a member of Facebook?
The answer may depend on the first language. The way you think of the situation. Prepositions are always subjective. Firstly know the connection between subject and landmark Here the subject is Sheila and landmark is facebook.
Sheila is using the facebook, not an integral part of facebook(meaning wheather Sheila is there or not on facebook, facebook will continue to work).
If Sheila is in facebook, then she is integral part of facebook. Here she is using facebook.Therefore, the correct form will be
Sheila is on facebook.
Take for example.
Cat is on the table.....(1)
But why not
Table is under the cat...(2)
Sentence (1) is correct because, we consider huger/heavier objects to be not moveable quite as easily as the smaller objects; and hence with respect to heavier object, we form the connection.
If sentence (2) were to be correct then use can say for the example like this
Facebook is with Sheila......(3)
Facebook is under Sheila......(4)
Both (3) and (4) are not correct.
Note that Facebook strongly requests that the only acceptable language for indicating that your business or organization page is active on their site is "Find us on Facebook". (This is to avoid the trademark dilution potential of phrases such as "Check out our Facebook page".) There is language inertia from "online" as well.
"Sheila is now in Facebook" would imply that she's inside Facebook, which could possibly mean she was inside their headquarters. (Example of this usage type). It would make no sense to be inside an abstract/digital concept like a website.
Likewise, it makes no sense to be on top of a website. But what you're really saying is "Sheila is shown on Facebook". Formerly, websites were static, and featured content on them. We've simply extended the usage for dynamic websites that you can join. "Is on" now means "has joined" or "is a member of" when used this way.