Last year we started a project (didn't work out) but we had a quit cool method to get emails and sign ups. In our case, we used the data of 500px - you can read about it here: http://saloon.io/geek-approach-to-marketing/
Even if you can't use the same strategy, maybe it gives you an idea for your customer field :)
Oh maybe we this isn't clear enough in the post, but we shut down our photo-sharing-tool (didn't work out - no retention). The "customer research" tool was only build for our photo-sharing-tool to find photographers.
Even if you can't use the same strategy, maybe it gives you an idea for your customer field :)