I think that one of the key things in the article that people a overlooking is the part about finding someone that you and the candidate know in common. He's not advocating just randomly calling people from the candidates LinkedIn connections, but trying to find someone you both know in common that you could speak to in confidence. My guess is the article got edited down a bit too much and didn't make this key point clear enough.
FWIW to the people who are commenting that they are highly turned off by this tactic... I know Brian Halligan and have worked with him twice (the last time being at HubSpot), he's a very professional and ethical guy overall (IMO) and I don't think he'd recommend doing something that put a candidates current employment in jeopardy. He is also not the only person I know who uses this approach. I've known several people to take this approach, and I've also personally been contacted by people who trust me, in order for those folks to get an unbiased back channel on candidates they were hiring.
It still doesn't make any sense. Just because Brian knows some person doesn't mean he necessarily knows potential problems in mentioning the candidates name (unless it's always Brian's BFF -- but I suspect it's really just some person who Brian happened to have some degree of contact with in the past decade).
Why does this need to be blind at all? If there's someone that Brian would like to talk to, just ask the candidate, "I saw from LinkedIn you know Sheila. I'm good friends with Sheila. Would it be OK for me to contact her as a reference, or would that pose any problem for you?"
And I think it's also on Brian's shoulders to make clear why Sheila provides values. "Sheila is someone I've worked with in the past and value her opinion." Brian should also give you the option of giving her heads up, otherwise Sheila may view the cold-call from Brian as you dropping the ball.
All in all, just seems like a bad idea. And frankly, I'd really like to see the data that shows he's gotten better hires from this.
The interviewing process would usually flow along a line of screening out candidates that are potential hires and THEN trolling their LinkedIn profiles to see who you both know in common. I think it would be tiresome to do that for EVERY candidate so that you can ask them during the interview "Mind if I contact Foo.".
As long you apply a little discretion about it I don't see what the big deal is. EG: don't contact someone at the candidates current company unless that contact really IS you BFF and you know they can keep the conversation private.
Also, if you do get any odd reviews you'd probably also want to give the candidate room to explain. I wouldn't have a big issue with someone contacting random ex-coworkers of mine as references. There is one person in particular though that probably wouldn't have good things to say, but digging further on that guy you'd realize he's a complete nutjob. So, you'd just want to make sure you properly tempered all the feedback, but in that scenario you should see overall trends and be able to spot outliers.
I guess I'm also not highly opposed to this because I think most current interview tactics suck as screening methods, and everyone hand-picks their references anyway, so you kind of need to go out of band to get a really reliable reference.
I'll also add that I think it depends somewhat on the level of person you're hiring. I don't know if Halligan still personally interviews everyone coming into HubSpot. If he does, I'm pretty sure he is not the one checking references for a level-1 phone jockey. So, we're talking mostly about the higher-order candidates, key individual contributors and managers that he's doing this for. It just doesn't strike me as such a deplorable practice.
Unfortunately it's difficult to guarantee discretion - for all you know the person you're contacting is the brother of the candidate's boss's girlfriend, for instance.
Furthermore, in my experience, references like to know in advance that they will be contacted: in my culture, any ethical reference is going to want to know who's asking, why, and whether they have permission to speak on the candidate's behalf.
Finally, there may be personal issues between the person you're contacting and the candidate that would make the reference biased.
Ultimately it seems pretty straightforward and professional to simply ask the candidate outright whether you can contact X, Y and Z as additional references (unless you happen to know these people well enough that discretion is relatively certain). At that point the candidate can say, "sure, but let me speak to them first to see if they're OK with that", or perhaps "No, Bob stole my wife from me and I hate that guy."
You say Brian only contacts people he already knows but the article clearly states otherwise.
"The hardest part used to be finding someone that had worked with the candidate somewhere along the line, but LinkedIn has made that easy. It will tell you who you know that is connected to the candidate by one or two degrees."
He specifically states he is contacting someone he knows that knows someone you know (two degrees). Not just someone you both know directly.
FWIW to the people who are commenting that they are highly turned off by this tactic... I know Brian Halligan and have worked with him twice (the last time being at HubSpot), he's a very professional and ethical guy overall (IMO) and I don't think he'd recommend doing something that put a candidates current employment in jeopardy. He is also not the only person I know who uses this approach. I've known several people to take this approach, and I've also personally been contacted by people who trust me, in order for those folks to get an unbiased back channel on candidates they were hiring.