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"),