As is the case with traditional SEO, getting links to your Facebook page and custom tabs is extremely beneficial. Links are like recommendations. If one friend recommends you eat at a certain restaurant, you'll consider it. If a number of your friends echo that recommendation, you'll be convinced.
If Google sees that your tabs and pages are being linked to by numerous sources, both on and off of Facebook, then your page and tabs must be important. But search engines also take into consideration where the links are coming from. A link to a custom tab on your business page will be weighted much more heavily if it's from a popular, recognized source.