Most online communities don’t succeed by prying masses of people away from existing communities. People in a successful online community are loyal to the community. Their friends are there, as are post counts, customized profiles and history of messages. These are hard things to leave behind.
Your new online community will comprise of people who have never participated in a community about that specific topic before.
There are some implications here. First, if the market is saturated with online communities, you need to do something different. You need to find a new group of people (more amateur, more dedicated, niche-within-a-niche etc…).
Second, the first organization to create an online community takes the lion’s share of the potential audience. If you’re the first brand to create a community in your space, it makes sense to build a community for the topic rather than your brand/products.
Finally, if you’re not the first, you can waste a lot of time by trying to imitate a successful community. Even with a better platform you’re going to fail. So if you’re late to the game, you need to focus on a niche topic (i.e. your brand’s products rather than the industry).