Whew, now that I have the space I can parse this out a bit better.
Your end goal is reaching readers, and I would argue that one does not need to toss in "community" or "potential community" into this discussion at all.
Reaching readers I presume means only internet readers.
For new readers:
This conversation boils down more simply to "how do the other internet marketers increase the discoverability of their website?" This is where the internet, a roiling mass of chaos with few central hangouts, requires you to go diffuse because you can do all the posting you want on your website, but it's one of millions (or hundreds of millions) and to increase your probability of being discovered by a new user means you must increase your footprint.
It's a lot of work
Sure, not all are great - but I'd say try at least a few places and look at the returns for your effort. I vote yes for Wattpad and "meh" for Livejournal (unless your friends list is still going gangbusters and you have a clear community to advertise to that is also still active).
If you are using Wordpress btw to craft your serial, you can crosspost to LJ and tumblr with the addition of one or two plugins. (If someone wants to know which one, I can check.) Or - have Livejournal import your RSS/XML if you're not using a Wordpress interface.
Community. What is it? What do you want it to be?
I think one point that has mystified me is the word "community" being used in one of the 1889 blogs and in this forum. This is where you guys have lost me.
Perhaps for those who have been talking about creating communities can explain to me how a member of a community is different from a fan? I actually don't consider people who are fans of my art or writing to be a community because their interactions are largely with me, not each other. Similarly, all my friends in real life do not make up a community simply because I'm a common element unless I help one of them start forming connections to another. Normally, though, this is a decision or circumstance that I don't control.(In that sense, trying to "create community" is a fallacy. You can only foster it or encourage it, but it's up to others to actually make it a reality. And one author or creator is not enough to do that unless you have really great people skills and a lot of time.)
Or was the original conversation about "community" really an offshoot of the idea that you wanted to create 1000 true fans ? In other words, getting to the perceived level of popularity one requires to make your creative work actually life-sustaining? (See http://www.kk.org/thetechnium/archives/2008/03/1000_true_fans.php )