There is a definite trend toward employing "Save the Date” announcements to be sure that the people closest to you have enough notice to plan their calendars to attend your wedding celebration. We think it's a great trend in our busy world.
Some things to note: You needn't send these to everyone on your list - after all, you probably don't even know yet how many people you will be inviting....That said - be SO, SO, CAREFUL, to only send the announcements to those people who you are ABSOLUTELY SURE that you will be inviting, and by all means - keep a list of who you sent them to.... It is forgivable to not have room for someone, but unimaginably rude to send a "Save the Date", and not follow with an invitation. So, as excited as you are...stop and think before you mail. But do mail.
If you aren't sure of everyone who will be invited, but still want to share your news, then send the card as an "Engagement Announcement" with no wedding date printed on the card, then add a personal note to "Please Save the Date of 00-00-00" for the people you are absolutely sure of.