In Safari?
Tap and hold in the URL bar, until a magnifying glass appears. Release, the choose Select All, then Copy. Go to FaceBook, tap and hold in the text box until the little menu pops up again, choose Paste.
In the body of the webpage you can usually tap and hold on the movie to get a pop up menu. Choose copy, then go paste it into FaceBook as before.
Many other apps and websites have share icons; usually a box with an arrow coming out of it. Sometimes they offer direct sharing to FaceBook. Others may offer email. In the case of email, go ahead and try it. You can copy the URL from the email, then cancel and delete the draft. Again, go to FaceBook and paste.