Search engines, while extremely helpful for people searching for information quickly, do not promote a sense of community. They offer the same sort of dynamic as using an encyclopedia - a dynamic in which the end user is not required to give back to the knowledge base and has to accept the validity of its contents at face value. In this type of top down knowledge delivery system, the effects of social capital is almost completely negated.
In communities such as the WELL

yahoo groups, usernets, mailinglists, forums, estrip you get to know the people who are producing the answers and can decide if you trust that person as a source of information based on their history and your personal interactions with them. With search engines none of that exists. There is no need to be accurate, only a need to show up at the top of the return list, which does not equate validity, just a repetition of keywords and other people (possibly yourself) linking to you.
Furthermore, the results of most searches on the Internet only give you information that already exists while online communities allow the user to pose questions that possibly have not been answered yet and get results from other people with expertise in that subject matter.
This is not to say that search engines are not the one of the best ways in which to find information, just as an encylopedia is a great way to find out about something.