When it comes to technology start-ups, there are a lot of paths to success.

Some successful start-ups sell for a billion dollars, as Tumblr did this year to Yahoo. When that happens, it’s a windfall for the founders of the start-ups as well as for the investors and some employees.

Other new companies are considered successful if they attract millions of users and venture money before going public, as Facebook did and Twitter is doing. And still others are considered hot because their products attract demographic groups that advertisers want to reach or because they have come up with some new idea or technology that seems to change the way we live.

About a year ago, right after Instagram was snapped up by Facebook, a few other start-ups seemed poised for some degree of success. Some of them, like Pinterest, Square and Uber, are still thriving, with plenty of users and cash from investors. Others, like Path or Airtime, have lost steam, and users’ attention, and may be on their way to the tech boneyard.

This year, there’s a new class of interesting start-ups to watch. Some are attracting record numbers of new users and significant amounts of venture capital, or appealing to a new and interesting demographic. Some are simply working on a new idea that seems to be taking off. In the unpredictable world of tech start-ups, any of them could fizzle and be forgotten by the end of the year. But they could also be the next big thing.