I could be wrong here, but it looks to me like .Site.Social.facebook is used in tpl/template_embedded.go, but the variable is never set. I've added a line to initializeSiteInfo to map the info from config into this variable.
                BaseURL:               template.URL(helpers.SanitizeURLKeepTrailingSlash(viper.GetString("BaseURL"))),
                Title:                 viper.GetString("Title"),
                Author:                viper.GetStringMap("author"),
+               Social:                viper.GetStringMapString("social"),
                LanguageCode:          viper.GetString("languagecode"),
                Copyright:             viper.GetString("copyright"),
                DisqusShortname:       viper.GetString("DisqusShortname"),