why i can't make a video call in Facebook

If you have an iMac PPC, it likely is because they are using too new a video standard. PPC refers to the processor in Macs from 2005 and earlier. Is that what you have?

Apparently TenFourFox 5.0 supports video chat on Facebook on PowerPC Macs. I found a post from someone who no longer contributes to Macrumors, a speculation page. But there is this Facebook page if that's what you have, where you might be able to find more info: http://www.facebook.com/macpowerpc