Linkedin for a B2P product. I posted a few questions asking if anyone had similar issues, and got directly in touch with about 20 people who responded to involve them in customer research, then kept in touch with them as I was developing the product for feedback. Those 20 by word of mouth led to a bunch more people trying it out. It's difficult to put a specific number on whether that was 100 or a bit more or fewer, but a few cycles of word of mouth from happy users got me over the 100 users easily.
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