1) All buttons in all sites linking to same root url to main site (increases counter faster) 2) Leave blank (no idea what the effect is) 3) Put the root of every site (?) 4) Link to facebook com / yourfanpage (gain likes faster)
The downside I see in option 4 is you gain fans, but you dont spread your website content. In particular I have and mantain better information in my sites, and don't reply every comment on facebook. Is better for us to keep traditional channels to get in touch with interested people or costumers (our website, our email our phone) instead facebook.