Going to individual academic institutions may be one way, but this would also be quite localized, and hard to scale. It also misses intelligent people who are unable to afford to attend such an institution, especially in a place like the United States where the cost can be prohibitive for many, even with scholarships and other aide.

Reddit probably isn't a good place. There's more snobbery and faux intellectualism there than there is actual intelligence, and that's even when focusing on the more intelligent subreddits. It's similar for those who regularly contribute to Wikipedia. It's more about politics there than it is about intelligence.

Digg is pretty much dead. Slashdot is nearly dead. HN has a rather limited audience. A lot of intelligent people voluntarily choose not to use Facebook, Google+, Twitter, or other social networks due to privacy concerns.

So 4chan does look like a reasonable alternative. There's a large audience, its members are probably smarter on average than you'd find elsewhere, and it avoids many of the problems with the other websites or social networks that put a lot of focus on identity.

With correct targeting you can get some very smart people.

Sturgeon's Law (90% of everything is shit) applies, and since /b/ is so rapid there is a lot of stuff in that 90%.

When /r9k/ started (I remember it when the Robot image was up) it was interesting. There are a couple of smart people in /g/ (especially if you're interested in Plan9 or FDE under Linux to protect Loli)

There is a bit of a problem with semi-smart people thinking they are smarter than they are. (And because of the volume of people you tend to meet a lot of them).