scroll on the new Facebook Chat,Facebook recently launch new design for chat on facebook, and it has effect that you can not scroll your friendlist chat.it is terrible.The new chat eliminates scrolling and instead displays all contacts in one shot.i think They need to fix it very soon because everyone hates it!
it is just tips so you can chat with your online friends on facebook, try to install facebook chat application, it is third party,using api to chat, for example you can use yahoo messenger and integrate your facebook account on it, there are many chat application facebook you can use it.
see the screenshot for many third party application which has facebook chat