Timing is everything when it comes to sending out invitations: Give people enough advance warning to fit your occasion into their schedule, and ensure that nobody feels they are an afterthought by sending all invitations out at the same time. You also need to consider how to approach the etiquette of using email to issue invitations. Social networking sites are generally considered inappropriate for invites.
It's All in the Timing
More formal occasions require more notice. As a rule of thumb, send invites out for a Christmas party or bar mitzvah about one month in advance. Invitations to a ball should be sent out from between six weeks to three months before the event. For less formal occasions, such as a dinner party, use the time frame of a few days to two weeks as a guide. Last-minute invitations are fine for spontaneous occasions, such as a bon voyage party if nobody has organized it in advance.
It is lovely to receive a formal invitation through the post. However, for less formal invitations it is generally easier to use email. When sending an invite out via email, it is good manners to make the message seem personal to the recipient. Therefore, don't send an email to one friend and carbon copy it to the rest of your guests, or write the invite and include all your guests in the "To" line.