Is reciting Al-Fatihah permissible in Islamic law when entering online forums, and is it permissible to participate in that?

There is no harm in participating in forums that call for reciting Al-Fatiha or sending blessings upon the Prophet, peace and blessings be upon him, or other forms of remembrance (dhikr) and worship. This is a good matter and an invitation to good. The Messenger of Allah, peace and blessings be upon him, said: "Whoever calls to guidance will have a reward like the rewards of those who follow him, without diminishing anything from their rewards." Participation must be with the intention of worship and drawing closer to Allah.