According to analysis from ‘social media scientist’ Dan Zarrella, the best time for publishers to send articles via Facebook is the weekend:
While I found less articles posted on the weekends those stories that were published on the weekends tended to be shared on Facebook more, on average, than stories that were published during the week. The reasons for this probably include the fact that more than half of companies in the US block Facebook, so people can only use the social network at home, on the weekends. Additionally, the mainstream Facebook audience does not use Facebook for work.
There’s more on his methodology at this link.