When you get right down to it, all marketing is finding an audience that someone else has, and borrowing it. If I'm buying AdWords ads, it's Google's audience, if I'm getting press in the NYT, it's theirs, if I'm putting up billboards in the middle of London, it's the M4's traffic into the city.
The key is threefold: 1. audience interest & profile compatibility, 2. clear conversion points, and 3. a UVP.
Breaking those down:
1. What you're pushing out, and the interests of the audience, have to be aligned. So if you're launching a new HN-style area, you need to know who your audience are (probably dissatisfied HN-ers, some people from Reddit, programmers etc), and identify where those people are (HN, GitHub, Engadget, Bit-tech, Twitter...) and how you can reach them (ad platforms, publicity on blogs that they read, referrals, recommendations from trusted sources).
Understanding your users is key, both in being able to find them, and being able to serve them when they reach you.
2. Know what your points of conversion are. Is it getting someone on to the site? Is it having them sign up for something? Registering an account? Downloading something? Leaving comments? Starting threads? All these things are valid conversions, but it's important to know which ones you want to look at, and which ones are just going to be distractions.
Also, what are you going to do with this data? How about setting up a notifier for your first 250 users, when you reach that point, so you can email them all in person and thank them for coming along? Think about what you want to do with the data you collect, before you collect it. Data without purpose is just time invested for no gain.
3. What's your UVP (Unique Value Proposition)? Why should people come to you instead of wherever else is available? Audience quality? Signal to Noise ratio? Curation of content? Design? Features? What's the benefit that you offer over someone else?
If you can't convey this clearly, you're going to have problems. It's rare to get something like Twitter's early days where downtime and a confusing message is overcome by audience enthusiasm and the community doing your messaging for you. Ensure that you know what you want to tell people, and how you're going to do it.
Hope this helps.