So this was just posted as a threat to Seattle Area Mosques.
I asked the Seattle bureau of the FBI about this. Spokesperson Ayn Dietrich said the post contains "threatening language" and the bureau is looking into the possibility of a related physical threat. She said local police departments with mosques in their jurisdictions have been made aware of it, and that collectively they are "maintaining a routine but vigilant security posture." An SPD spokesperson said they received word from the FBI about it yesterday.
The Facebook account—which shows a number of its posts originating from Edmonds and Seattle—appears to have deleted the post referenced in the above tweet. Today, a newer post on the Facebook account said: "A lot of people have claimed that I have made threat to 'shoot up' local mosques. I have Not. No such action has ever been mentioned on this page. Again you people lie to suit your own agendas."