Let's start with the big picture.
If you're keen on astronomy, astrophysics, cosmology, etc and if you like to keep up to date with developments, and if you like astronomical images, of galaxies, stars, planets, etc ... then you have rather a lot of websites to choose from!
There are blogs, like
Bad Astronomy and
Cosmic Variance, both on Discover. Now a blog is someone's (or someones') post, followed by lots of comments. And there are rather a lot of physics blogs, most of which have some blog entries that are interesting to space fans (one of my faves is
BackReaction).
There are news sites, like
Universe Today, which allow comments on each story.
There are discussion fora (forums), like
BAUT, which is very general, and
GalaxyZoo forum and
Einstein@Home Message Boards, which are mainly project oriented.
Some sites, like APOD/Spaceship Asterisk* and
Space.com, combine "news" with "discussion".
It seems that every one of these develops a community after a while, with a smallish number of high post-count members, a smallish number of respected/trusted members, a lot of irregulars (low post-count, but they don't leave), many butterflies (come for just a while, then flit off elsewhere), and many, many lurkers.
Interestingly, most discussion fora, even highly focussed ones, have 'Just Chat'/'Shoot the Breeze'/etc sections, which may end up with more posts & views than the 'serious' part of the forum! Forum members are, it seems, very social creatures.
In terms of hits, I think APOD is by far the leader, as can been seen by the dominance of APOD pages in 'popularity' sites like Digg.
In terms of community, I think BAUT is probably the leader; however, once you break it down at bit, the picture is less clear; for example, Physics Forum's
Astronomy & Cosmology section is really the only place to go for high school/undergrad (and some postgrad) level technical discussions (it's fascinating how badly technical threads flop in sites like BAUT).
So, in terms of RJN's question, it seems it comes down to what makes for a good, biggish community?
And, more narrowly, how to 'keep' those who, somehow, drop by? (How to 'fill the funnel' - get more people to visit in the first place - is a separate discussion).
Here's one input:
JREF's SMMT section attracts huge numbers of posts and views. And, in astronomy etc, by far the most popular threads are those in which some, um, 'alternative' notions are vigorously promoted (and trashed). I once thought this was a shame; since I have learned that many of these threads are sterling examples of the 'E' in JREF's name ('educational'). It seems that lurkers are legion, and that they become, in their own words, much better educated about, say, LCDM cosmological models than they would if they'd simply visited
NASA's Ask An Astrophysicist, say. The same thing has happened with
BAUT's ATM (Against the Mainstream) section, despite the enormous pain that section causes the moderators.
Well, that's enough from me, for now.