When do you know which DOTs to connect?

From the beginning of this semester, it has been one talk or the other about social networking. This includes the various ways that social networking has been helping organizations and people in general to connect. This weekend, I met some classmates from high school. We had lost contact for about 8-10 years and we only got to hook up through Facebook. Reading Business Networking Advice by Josh Hinds has made me better understand the question he asked Ryan Mapes, one of the people behind GoBigNetwork about mistakes made in business networking. According to Ryan,

the biggest mistake I see is a lack of follow-up after meeting someone. If you meet an important contact and exchange business cards, be sure to send a quick email when you get back to the office. This can help solidify the relationship and open up the doors for further correspondence down the road”

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This to me doesn’t apply just to the business world. It is an important part of the social network concept to bring people together to share ideas, thoughts and interests.
Social Networking has opened a lot of doors for people. There have been a lot of successes recounted by people. An instance would be the article Six Degrees of Cooperation by Yasmine Ghahremani where Andra Marx a senior account at Intralinks strikes a deal thereby making millions of dollars for the company. According to her

“the deal involved some fortuitous timing, but it certainly wouldn't have transpired if I hadn't been able to connect the dots”

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My question then is, how do you know which dot to connect? As in whom out of your social network group should you keep as a serious contact or not? Writing this post has made me remember my experience on Facebook which I posted on my first blog post when talking about stalkers and privacy. Can a person like the one mentioned be a dot for me?