When Facebook bought Instagram for $1 billion last month, it raised a lot of questions about which buzzed-about start-ups might be on track for similar success. The start-up scene is flooded with apps and services that are attracting users and backing from investors. But it can be hard to work out which companies are worthy of the kind of attention Instagram was receiving when Facebook came calling. Here is an inherently incomplete list of companies that have the potential to be a hit -- whether because they've seen explosive user growth, or are attracting investors or a new demographic, or just because they have an unusual idea that seems to be taking off. Of course, any of these start-ups could become the next Pets.com. But they could also be the next big thing.