What is the ruling on publishing and writing the phrase "If you love Allah, then write 'La ilaha illa Allah'" in forums and on social media?

There is no legal prohibition in reminding people with a voice; rather, it is a form of assistance in the remembrance of Allah. However, it is more preferable to remind them of the virtues of various forms of remembrance (dhikr) and supplications (du'a) and to encourage them to perform these. This was the method of the Prophet, peace and blessings be upon him, and it is more beneficial.