I thought bhikkhus were to not to be involved with entertainment. Thank you in advance.
I don't think their intention is to entertain but to to teach.Social Networking Sites can be used as a form for communication.Especially in this day and age.It's likened to them writing a letter or giving a dhamma talk.Since i don't attend any Buddhist monastery or retreats or groups,I have benefited from monks teaching the Dharma on Youtube or contacting a monastic via facebook when i had any questions etc.It is also a way for the monastic to keep in touch with the lay people.So intention plays a big role in why monastics use social networking sites.
Youtube/Facebook etc. are just tools... one doesn't watch "youtube", one watches a movie, a vídeoclip or a Dhamma talk for example.
Think in youtube as a simple channel of communication, if you use it right, watching, let's say, Dhamma talks or guided meditation, how can that be bad or breaking a precept? Same thing goes for facebook or blogs...