the social classes meme over at hardcore: http://spn-hardcore.livejournal.com/570878.html

Well that's a really broad request, because prompts can come from just anywhere, but they're often found on comment-fic memes.
For starters there are pure meme communities like (spnkink_meme or blindfold_spn, and to include one that won't make me look like a total perv sharp_teeth which is multifandom but with a lot of SPN), but they're also pretty common on other communities (like on ohsam, hoodietime, or spn_hurtcomfort - to find them check (http://ohsam.livejournal.com/tag/%26comment-fic%20meme) the (http://hoodie-time.livejournal.com/tag/.comment-fic%20meme) tags (http://spn-hurtcomfort.livejournal.com/tag/type%3A%20meme)) - and sometimes they just happen on people's journals (e.g. here (http://roque-clasique.livejournal.com/58567.html) or here (http://fleshflutter.livejournal.com/153897.html)).

Just to give you a few that I'm aware of, but there are many, many more (probably also some that focus on J2, but that's not my genre).

The Classic Canonverse J2 Comment Meme (http://waterofthemoon.livejournal.com/497429.html)

You got this community: comment_away

The bottom!Jared Fanwork Fest (http://transfixeddream.livejournal.com/102658.html)

Let's Get Gay Married (http://bree-black.livejournal.com/84411.html)

Comment Fic Meme - Conventions (http://j2-a-day.livejournal.com/127245.html)

Awkward Date is Awkward (http://deirdre-c.livejournal.com/411710.html)

Working Hard For The Money - Take One (http://transfixeddream.livejournal.com/64707.html)
Working Hard For The Money - Take Two (http://transfixeddream.livejournal.com/94194.html)

The Schmoopy J2 Comment Meme (http://j2-a-day.livejournal.com/34813.html#cutid1)

That's what I can come up with. I know there's been one with prompts from LGMH, I would love to find that one myself :)

JDM and John Winchester (http://rozabellalove.livejournal.com/70137.html)
Domesticity (http://comment-away.livejournal.com/1536.html)
Hoodie Time Comment Fic Meme (http://hoodietime.livejournal.com/tag/comment-fic%20meme%20%28masterlist%29) (Masterlists)
Schmoop (http://comment-away.livejournal.com/831.html)
Hurt/Comfort Meme (http://spn-hurtcomfort.livejournal.com/5443.html)
JDM/Jensen (http://mysticwaters.livejournal.com/99378.html)
Holidays (http://longsufferingly.livejournal.com/65351.html)
AU Meme (http://dev-earl.livejournal.com/204730.html)
Prompt and Fill (http://spnroundtable.livejournal.com/tag/prompt-and-fill) on spnroundtable
Vacation (http://arabella-hope.livejournal.com/670770.html)
Sports (http://deirdre-c.livejournal.com/376250.html)
Fix It (http://lazy-daze.livejournal.com/626186.html)
Smith and Wesson (http://smithwesson-spn.livejournal.com/4359.html)
How the Winchesters got their groove back (http://wanttobeatree.livejournal.com/943667.html)
Dean/Castiel and Jensen/Misha (http://qthelights.livejournal.com/758207.html)
A Sneezy-SPN-Boys Comment Fic Meme (http://mad-server.livejournal.com/44195.html)
Meme (http://samdean-otp.livejournal.com/tag/comment%20fic) on spnroundtable
Meme (http://spn-hardcore.livejournal.com/tag/~meme) on spn_hardcore
Summer Lovin' (http://packolies.livejournal.com/12362.html)
Drabbles (http://longsufferingly.livejournal.com/121544.html) at longsufferingly
Take The Knot (http://take-the-knot.livejournal.com/5109.html)
Awkward Date is Awkward (http://deirdre-c.livejournal.com/411710.html)
Clothes Sharing (http://neros-violin.livejournal.com/14017.html)
The Kissing, Cuddling Nuzzling, Wrap-You-Up-in-My-Clothes Comment Meme (http://dante-s-hell.livejournal.com/154531.html)

Huh, so I hope the links are working! Hope you enjoy :)