Let's just say it was a video one of your relatives posted and you wish to edit it...
You can find the video you'd like, then in Safari Window/Activity you can find the .flv file Double-click on it to Download it.
Then find a program to convert the .flv file to Quicktime so it can be read in Final Cut. There's many out there but I use iSquint. Play around with the export options in iSquint to get the size and quality you want.
It
can be done, whether you
should be doing it or not well... that's another issue.