Wait a month or two (because you need to have good enough performance from one platform before testing new networks because it’s always an investment to start a new network and therefore a new learning period) and then launch a Facebook remarketing campaign.
Start out with images you find around your site.
If you have a large customer list, and if your Google account is eligible for "customer match," then you can implement some cross-selling and retargeting campaigns. Because the audience size is limited and much smaller than an audience found on Google, we won't need a large budget for this strategy. After about a month or two of gathering conversion data, we can also create a "similar audience" based on the customer list. Similar audiences are often effective, and we will incorporate it into our proactive targeting campaigns.