I've built a supposedly useful site. Now how do I make it known?

So for the past few months, I started building a web site that's targeted to a niche group of people. It's partially a social network.

My assumption is that it will be useful and attractive to a lot of people out there. I shared it with friends and they've been using it every day, so I've been getting good feedback.

I myself been using it since the very early prototypes, since it was solving a problem I had.

Now it's reached a point where I think I should stop with adding more functionality, and start somehow advertising it to people that might find it interesting and join.

But I'm a bit stuck as to how to market this and make it known. Do you have any advice on one generally goes about making their project known to the world (not anybody, just the people that might care about it)?

To clarify: I don't have any plans or even thought about monetization. I'm doing this for fun and I there are no subscriptions charged etc.

Thanks

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EDIT: I guess what I want to say is that... OK I've spent effort building this, and it looks good and useful to me. How do I validate my assumption that it will be good and useful to others?

Seeing people joining and using it would be a huge motivation to make me continue working on this. (Not that it's the only motivation.)

EDIT 2: Since this is for a pretty specific group of people and it aims to be free, I don't want to put paid ads anywhere.