Is not thanking people—who provide fatwas and useful websites—considered ungrateful to Allah? And how can one supplicate for them despite not knowing their identities or religion?

To show your appreciation for those in charge of the Fatwa Center, it suffices to make a sincere supplication for them in their absence and to speak well of the website, based on the words of the Messenger of Allah, peace and blessings be upon him: "Whoever has a favor done for him and says to the doer: 'May Allah reward you with goodness,' has indeed praised him sufficiently." The supplication of a Muslim for his brother in their absence is answered, and he receives the like of what he prayed for. Knowing the gender of those in charge does not detract from the supplication and praise. As for non-Islamic websites, it is permissible to supplicate for the guidance of those in charge of them.