One of my favourite examples of a successful online community is The Student Room.
The Student Room gets a lot right. Lets examine a few of the key factors
1) The Platform
The Student Room uses a simple platform that encourages interaction between members. It displays a lot of activity on the landing page while also having the option to display content articles. Many communities struggle to do both.
Most importantly, the platform is simple. You don't have to join to see all the discussions. You join to participate.
The Student Room has a consistently high social density across the site. They introduce new forum categories for popular topics and facilitate a high level of off-topic discussion. Right now discussions range from fitness, choosing universities, politics, religion and romance.
This is a community which has clearly reached the mitosis phase in the community lifecycle. It's sustaining high levels of participation from a relatively fixed-number target audience.
The Student Room engages in events and activities. The community is currently organizing a ski & snowboard trip for its members in 2012.
4) Strong sense of community
Students have a good sense of community anyhow, but the Student Room has excelled in introducing the elements which facilitate a strong sense of community. They have a good personality, shared history, clear membership etc...
The Student Room has monetized the site by providing value to members. They're picky about advertisers. This avoids the MySpace trap. The advertising is clearly apparent but not overwhelming. They have room to go further by producing guides and products specifically for their audience.
6) Wealth of knowledge
The Student Room has cultivated an unparalleled wealth of knowledge about their topic. All of which has been created by its members. There is a clear benefit from joining and participating in this community.
The Student Room helps students be better students. Not enough communities do this.
You can learn a lot about developing and managing successful online communities by looking at The Student Room.