Facebook provides 3 options, but only 2 of them are viable in Shopify.
HTML: Shopify doesn’t provide access to the root domain and you cannot add any files there
DNS: It is possible by adding a TXT entry to your DNS record. That could take up to 72-hours, but usually it is much faster- Meta-Tag: this will allow to almost immediate verification, but you need to customize your theme code. That is a simple operation and can be done inside theme.liquid file inside layout folder of your theme