Optimized for IE used to be what the cool kids did because it supported iframes before Netscape and all of a sudden it wasn't a big deal to use frames in that way. It took a while before div based layouts actually were easy to get working in most browsers to the point where most people stopped using Photoshop and cutting everything into borderless tables.
I'm not afraid to admit that I still do table-centric HTML design on occasion (disclaimer: for specific, usually non-public, uses); it's really not all that much worse semantically than the current trend of "oh I'll assign a grid-three-seventeenths-or-whatever-the-fuck CSS class to this div in WangularStrap.js", considering that in both cases you're embedding formatting information in the data being presented (and therefore totally missing the point of "semantic" web programming).
"Social media are computer-mediated tools that allow people to create, share or exchange information, ideas, and pictures/videos in virtual communities and networks." (Someone added "Web 2.0" later there but it's immaterial. Well, HN lets you vote without refreshing the page, I guess that's "Web 2.0")
The content is driven by users, either by creating (FB, Twitter) and/or curating it (Pinterest AND HN) -> Social Media
http://en.wikipedia.org/wiki/Bulletin_board_system ("A bulletin board system, or BBS, is a computer server running custom software that allows users to connect to the system using a terminal program. Once logged in, the user can perform functions such as uploading and downloading software and data, reading news and bulletins, and exchanging messages with other users through email, public message boards, and sometimes via direct chatting.")
HN is actually a fairly primitive BBS running over TCP/IP.
There's a massive difference between topic-centered, article-oriented forums and social-oriented "life walls", in which people are drawn to share their personal life and basically have an "online life hub".
One is centered around the topic, the other is centered around the person.
Reddit is topic oriented and nonetheless is social media. Because the curation of topics is done by the users (same thing here, with some content created by the users themselves)
Pinterest is about curation of content and it is social media as well
As I said, it's about the difference between "topic-oriented" and "person-oriented".
Pinterest has a very different demographic which is more likely why GGP hadn't heard of it, but I would put it in the same bucket as Reddit/HN. Facebook, Twitter etc however are person-oriented.
The only thing all these have in common is user-generated content. But at that point you can call anything with user-generated content social media. Forums? Sure. Newspapers with curated user-submitted articles? Why not.