Many people have access to multiple genealogy sites (e.g. Ancestry.com, Geni.com, MyHeritage.com, etc.). If you would like to extract data from different sites, you can do that simply by selecting them in the drop down on the Assign People page.
Although you can extract people data from different sites (including Facebook), you cannot populate your pick list of people from two different sites at the same time. You’ll need to access one genealogy site, assign the people you’d like to your cards, then select a different site and assign those people to your cards, etc.