A form of Appeal to Popularity combined with Appeal to Consequences; here, it's suggested that because something is becoming popular, it should be accepted quickly or the person being spoken to will lose out in the long run. The name comes from the classic idea of getting on the bandwagon before it leaves; in this fallacy, the fact that there are a lot of people on the bandwagon and it might leave are the only reasons given to accept, with no reason why getting on the bandwagon is actually a good idea (or, for that matter, why there is a bandwagon).
This is referred to as Fear of Loss in sales; a salesman will claim that he's only allowed to sign up a certain number of people to a fantastic deal and has already got most of his quota for today, so if the person he's speaking to doesn't act they stand to lose out.
There are also times this argument is valid, such as when there are what economists call network effects. In brief, if the value of a good or service changes based on the number of users, then pointing out the number of using it could be valid. For example, when telephones were adopted, their value increased with every new telephone added to the network. If business software is used by many companies, being ubiquitous is a selling point. If no one else uses an instant messenger, it's useless, but if everyone uses it, it's more valuable to the end user. If all of one's friends use a specific social networking site and you want to use social media, it makes sense to follow your friends. If a cell phone company allows unlimited calls between two members of their networks, the number of clients they have and their demographics are both legitimate concerns. If most counties and companies are using a particular shipping container, rail-road gauge, or standard of measure, there's good reasons to adopt the same standards.
A standard technique for infomercials: "The next fifty callers" will get the deal being offered, or the offer is only good "until midnight!" despite the fact that the infomercial is going to run for the next two hours, and tomorrow, and the day after that... It's not at all uncommon for the infomercial host to overtly admit they're using this fallacy by saying something like "You don't want to miss out!" or "Don't be the only one in your neighborhood who doesn't have <whatever it is>."
Franklin Mint and other so-called "collectibles" companies make a point of stating that there are only a limited number of whatever gewgaw they're selling, and that "When they're gone, they're gone forever." Often the limit is a number of days that the item will be produced (for instance '300 pressing days' on commemorative coins, or '500 firing days' on decorative plates) with the hope being that the would-be buyer won't realize how many hundreds of thousands of coins can be minted in 10 months or plates can be fired in 16 months.