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Nobody will come to your new website if they don't know it's waiting for them.
Buying ads from Google and Facebook is one way to start, as you only have to pay for the clicks you get.
Because a popular website is going to cost money to operate each month, if you want to keep your service free, you will have to find other ways to monetize it as it begins to grow.
One solution is to run ads with a service like Google Adsense, or by offering ads through an affiliate arrangement for companies interested in your niche, making commissions for each sale.
If your niche is for people 50 and over, an app may be optional, at least in the beginning.
Finally, you might want to consider a freemium model.
Basic memberships are free, but if clients want to send unique emoticons, or share personal pictures with people in chat, they would have to pay a monthly fee with a premium account.
For any online business, your domain name will be your most important asset. If you're going after a particular niche, you can incorporate that into your name.
Just keep in mind that if you plan to go to other niches you'll have to get additional domain names.
You'll then have to upgrade to a dedicated server, which will cost you much more.