Research and the subsequent writing of papers uses up a lot of time and ressourcesresources. If by promoting your research after publication you increase visibility, readership and impact of your paper, it means a more effective use of that ressourcesthose resources.
But chosingchoosing the right channels is key: professional and academic sites like LinkedIn or ResearchGate are probably a better place to promote your research to peers than facebookFacebook, where your paper announcement might get lost between cousin Joe's wedding anouncementannouncement and an addad.