Oddly enough, some time ago I posted the needed addresses. It is down the list from here,, and I wrote some weeks ago too. I think that both should be done, both emails and snail mails. Snail mail is counted more though. It is getting to be more a rarity, but it should be done. I think the jury is still out as far as SFC is concerned, and that it said Season ender was a good sign. But, write people write, and it cannot hurt.
Yes, I'd highly suggest using snail mail and not email to show your support. Email really isn't counted as seriously at all, from what I've been told. A real, tangible letter shows effort on the viewer's part and has more chance of being seen by the right person. As quick and easy as it is to send off an email, it can be deleted and forgotten just as quickly. if you can do both then that's great but if you can only do one, then do try a real pen and paper.
If you've been buying the episodes off iTunes, make sure to buy them all now, too. I think if SFC/Lionsgate see that the episodes are still selling well after the broadcast as finished they will look at it as growing/positive word of mouth about the series.