Is it permissible to use a Facebook account that was purchased from another person who no longer uses it, or must it be returned to the seller or closed, after attempts to contact its owner were unsuccessful?

The default is that buying and selling accounts is permissible, and mere doubt is not to be heeded. However, if you are certain or it is highly probable in your estimation that an account is stolen, then it is not permissible to buy it, and it is obligatory to return it to its owner and claim the price back from the seller.