I believe online communities can replace things like synagogues and churches. For people who follow religion for the benefits of community over everything else, this could be a viable option. I believe on-line communities can become very close knit and they can also be very supporting. You get what you put in.
Many people attend church, synagogue, or other religious services for the human contact. They want to be with people who share similar values and experience their religion in a social setting. This simply cant be done over the internet. Sure, you get some sense of community, but its not quite the same as being in the room with other people.
Certain things like mikvah can't be done online. Likewise, the Christian church has baptism and other things that can't be done online. Online communities are certainly a great idea for those that are unable to attend traditional services or may feel out of place there, but they can't really replace them.
Online communities will never be able to provide the support that a congregation that meets at a religious building will be able to provide. Churches are not only used to spread the word but also to provide support systems and to form lasting relationships within the community. Many people rely on these relationships to not only get by in life but to help them when they are in need. You relate more to someone in your community then some one you meet online.
People who like to go to things like synagogues and churches to worship will not find online alternatives to be appealing compared to the real thing. While online services can be used at times, people will still rather go to an actual synagogue or church to worship at for their faiths.