"What's the best way to communicate with your downline?"
There are more ways to communicate with your downline u can use them all,any way can be helpful for your downline i think the best way is to communicate with them through the SFI teammail,then u are sure that they get your message or through the SFI chat when they are online you can find them in your Genealogy here
https://www.sfimg.com/Reports/Genealogy?lg=t