What is the ruling on the money I initially received from a fictitious company, and the money I later took from my friend in exchange for selling him my account, knowing that I am unable to return the money to its original owners?

Paying the subscription fee to register on browsing sites is impermissible. The transaction is فاسدة (corrupt/invalid) and must be annulled. You must return what your friend paid you, and there is no harm in accepting his voluntary waiver of a portion of the amount. You are entitled to claim back from the owners of the fictitious company what you paid them, and what you received from them is part of your right. You are also entitled to a fair wage (أجرة المثل) for the period you worked.