I'd think that people would choose the professional connections they trust rather than their stoner roommate from college, but there's only so much hypothesising we can do without real people trying it out!
I'm not a huge fan of Bob Marley either, but I know what you mean. It could be your friend who is, say, a prominent Rubyist tweeting a joke about PHP as you're applying to Facebook. We'll see how that plays out, but I'd think that an 'old school' job reference would have the same risks.
That's an amazing idea — we hadn't thought that far ahead yet in terms of NLP stuff, but we can definitely do some amalgamation of Twitter and, say, GitHub data to help you know who to target :)
The idea is that you'd share the link privately just with people you really trust, so it's unlikely someone you don't trust would add their recommendation. I agree - you probably need to be able to remove any recommendations that you don't think are appropriate...
The same correlation (or guilt by association) could be made by the people you follow on Twitter or your Facebook friends or the people who endorse you on LinkedIn.