Community websites - what is the secret of success?

Today in the E-Marketing chat we have in office we talked about E-Marketing Strategy and how community building is one such strategy. We discussed by taking examples from the real world - especially from Sri Lanka...

Few popular social sites in Sri Lanka we discussed
elakiri.com - Started as a songs downloader for Sri Lankan music fans. Now traffic rank #1 local web site (alexa.com) in Sri Lanka. Glad i've met the creator at the Java Exhibition.
erbenizer.com - One of the best designs. Awesome writing. Is branding to be the ultimate youth Magazine. Catering the popular school crowd (Well - yes.Im also one of them.lolz).
thepapare.com -  Novel idea. Started to discuss College sports. Now have 5000+ fans. Im not favoring my classmates Feroze and Hafeel who were involved in initiating papare idea.lol.
technohug.com - Started by one of my teachers - Malinda Alahakoon as a network in Ning. Is addressing a focused area of technology young crowd of Sri Lanka.

We discussed how our own yooth.com is not doing so well. I have few theories for a successful community site in Sri Lanka.

1. All successful sites have their own physical crowd or presence involved.
For e.g.
papare - has the sportsmen, fans who spend Saturday evening watching a match..etc. Its great if there is a place where they can chat about happenings in other 6 days.
erbenizer - The college crowd needs somewhere to talk about al lthe parties,events..etc.
Technohug - The guys who learn Java in technohug, IT students of Universities in SL, BIT Students and those who read for BCS in eSoft are the target market. Just go and see - they talk about simpel Java programming to ROBOT programming there. They might have their lectures, classes once a week - And technohug is the perfect place to meet in other 4-5 days.

2. Successful sites address one critical need of members (Focused product, focused market segment)
Elakiri - Music fans need to listen to their fav.songs
Ananmanan - Music fans need songs, lyrics.etc
Papare - Address Sports fans in Sri Lanka.
Technohug - Address hi-tech students.

More theories are welcome...