Complex sentence: Immediately, a Facebook group promoting a people's gathering on that day was founded; the service blocked it the next day following an order by Roskomnadzor, with the count of those who announced they would visit it approached 12,000.
Split into simpler sentences: Immediately, a Facebook group promoting a people's gathering on that day was founded. The service blocked it the next day following an order by Roskomnadzor; as of the moment of the blocking, the count of those who announced they would visit the gathering approached 12,000.