The NY Times has an interesting article on throwing a Facebook party and who you can expect to show up. The reporters analysis lines up with what I have noticed about Facebook events.
When invited to a Facebook event, you have three options:
1. Definitely attending
2. Maybe attending
3. Not attending
Translation of these responses:
Definitely attending: You can probably expect them to show up. They are interested, but something else could come up. Definitely translates into likely.
Maybe attending: They probably aren't coming, but don't have the heart to click 'Not Attending.' Your party might be one of many choices for them. Maybe they are seeing who else is attending to convince them one way or another.
Not attending: This is pretty straightforward. They proactively let you know they aren't coming. They are either too far away to attend or they really want you to know they have no interest in an event that you worked so hard to plan.