What's your product and who is your target user? Where do they spend their time? LinkedIn, Reddit, and HN won't help if you're trying to reach, for example, teenage girls.
TL:DR;
- get a splash page up and start collecting emails
- guest blog - particularly in places you know your would-be-users will read
- organize an event
- play around with paid marketing
I collected some thoughts on this that you might find helpful: http://lgilchrist.github.io/how_to_get_your_first_100_users/
TL:DR; - get a splash page up and start collecting emails - guest blog - particularly in places you know your would-be-users will read - organize an event - play around with paid marketing