Comments allow content to be added to a post. If you can get a healthy comment string going by people who are not trying to spam your comments (like many do) you are adding FREE content to your pages. The healthier the discussion, the more value being added to that page for FREE. This is of course if your comments are followed. Make sure you enable the ability to moderate comments so you dont have bots spamming at will.
I cannot speak about trackbacks but I've enabled them and have not really seen much come from this. Comments also create transparency. Facebook is great for giving your business dimension but keeping this type of discussion onsite is more beneficial for your site.
There is also a 'freshness' factor in the algo. In fact I've read some in organic circles lately say the freshness factor is cranked too high and freshness can give you too much of an edge. (hint hint)
So I would think comments add freshness. You posted an article last week BUT folks are still adding comments this week and next, that's additional fresh content.
I would think it's also a signal of popularity to the algo.
Trackbacks are fine and can help encourage links. As long are you aren't just getting spammy trackbacks. BUT you can't really control that. Even if you didn't allow trackbacks that would not prevent spammy sites from linking to you if they want to.
Another important thing to remember which Linda mentioned, you have no control over who links to you. Google cannot (I guess used loosely) penalize you for inbound links unless they look like they've been purchased. Paid links, link farms, spinning content, etc. may get you caught up.
Linking out to bad neighborhoods is what can get you into trouble.
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