I don't like answering my own questions, but just to get the ball rolling, let me mention a few that got me thinking along these lines:

• Thanks to User-13743967034596023228 I just discovered tvtropes.org (yeah, I know, very late to the party) and was amazed at how I'd missed such a major subculture. Now I have multiple people warning me to stay away since it is apparently a serious addiction risk.
• Wikipedia is definitely an "InterWeb" in its own right. I am not sure if Wikia belongs in the same InterWeb or is separate.
• Within the broader blogosphere (which is too diverse to be called an "InterWeb" subculture), LiveJournal definitely is a distinctive one. I THINK it intersects strongly with, but is not identical to, the fan fiction InterWeb.
• 4chan is another tvtropes like InterWeb that I discovered very late (and still don't understand at all)
• The Well http://www.well.com/ is obviously a major historical one.
• I don't think Facebook or Quora or any InterWeb dominated fairly dictatorially by a single corporation counts, since that makes the edges insufficiently blurry, and they are more like infrastructure providers than cultural catalysts, but I'd be interested to hear counterarguments. I think Facebook now is big enough that subsets of it might constitute distinct "InterWebs."