At a glance, a product like this seems obvious but so many have tried and failed. They've failed, IMHO, because they fail to capture enough value for customers to change their existing behavior and social norms. In fact, going to the bar to get a drink, even if the line is lone, isn't necessarily a bad thing. It's an opportunity to escape a conversation or pull someone to the side to chat.
Here's a fantastic postmortem on Flowtab by @alex http://techcrunch.com/2013/08/31/the-decline-and-fall-of-flowtab-a-startup-story/
Agreed @rrhoover, on top of that these are tough products to build. they require a lot of integration and reliance on 3rd parties - most of which are technical dinosaurs. Innovation is slow when you get bogged down by others.
@TabbedOut is doing this as well as anyone, but I know they have had the same struggles....
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