The fundamentally different aspect is also at the user end - you need fb account to run these gadgets, spend money associated with this account and when someone

The fundamentally different aspect is also at the user end - you need fb account to run these gadgets, spend money associated with this account and when someone tries to “hack” that account (because it is public), you loose it without a chance to ever get back the account or money. There is no way how to contact any living person for support, most you can get is their faq. I cannot imagine such situation with gmail.