FOMO is a new acronym which stands for 'fear of missing out'. In a way this has always existed - nobody likes to feel that all their friends are in on something that you have missed yourself - but it is much stronger now due to modern media.
In the last few years it has become possible to remain in constant touch with your friends and acquaintances through Facebook, Twitter and and an ever-growing list of other social networking sites; and as most people now have a smartphone or other hand-held device, you can keep updating your status and checking your contacts everywhere you go.
This can be great, but it can also give you the feeling that the whole world is having a big party and you haven't been invited. There are stories about some people even taking pictures of themselves and putting them on Facebook to make it look as if they've been having an exciting social life, when really they have just been staying quietly at home.