Unless your wedding is on a holiday weekend, or a DW, I wouldn't reccommend doing a STD. It definitly locks in your guest list way ahead of time, and who knows what unexpected events may come up in between that time and the wedding in which you have to exclude guests.
As for Facebook, if I was doing STD's that's not something that I would consider, but it could just depend on how your wedding is going to be. Is it more casual, or more fancy? If your just doing something small and having like a BBQ then I think it would be fine, but if your doing it at a banquet hall, hotel, etc, I probably wouldn't. That's just my opinion though. You really can do whatever you want and whatever you feel is best for your budget, guests, and how it all fits in.
I just met with my wedding planner and talked about this same issue. She said save the dates are just an option. Use them if it is a holiday weekend, or if a large majority of your guest are from out of town or if there is a large guest list. She suggest that we just set up a wedding web-page and just tell people word of mouth about it. As we inform people about our engagement we will be letting them know about the web-site. Also, a web-page is a good place to mention where you will be registered.
I'm going to let our relatives from out of town know the date well ahead of time, especially those who would have to fly. If anyone asks, I'll tell them, but I'm not going to send STDs. Saves time, saves money, saves paper, saves headaches of locking yourself into a guest list so early.
Personally I don't think the facebook idea is a very good one, because there could be people whom are not on your facebook but talk to some that are. They could get the idea that perhaps they're not invited because they didn't get the annoucement about the date. kwim?
As mentioned, unless you're getting married around a holiday I wouldn't worry about sending out STDs.