SDWAN PSLA for facebook failure/failover?

[vlw38]

Testing SDWAN failover between Internet (wan1) and MPLS (wan2) interfaces on 60E/6.0.8. Fortigate interfaces are connected to cisco switch. Failovers successful if I shut down MPLS or Internet switch interface connected Fortigate. But… I’m trying to simulate failure to let’s say facebook.com via WAN1. Configured loopback interface on switch to simulate facebook.com and created PSLA Test1 to monitor this loopback. I shut loopback interface, PSLA shows red down arrow but failover does not occur. Perhaps this is not the correct approach..how would forum-folks test/simulate this type of failure of lets say facebook.com via WAN1? Or, how would/could you configure SDWAN rules/PSLA's to handle such a failure?