There are about 21+ different Yahoo groups for every neighborhood in Brooklyn. The Yahoo Group I belong to is called Park Slope Parents and was started in 2002 for "...local parents could exchange information via email/bulletin board. Parents discussed a broad range of topics, from "does anyone have an idea of how to get a baby to take a bottle?” to "where's the best place to get shoes that fit?” and “can anyone recommend a person who could come and childproof my house?" Now, more than 10 years later, 5,000 families are members, using the two three large lists -- Advice, Classifieds and Career Networking as well as our pregnancy and baby groups, bilingual groups, 'tough stuff' groups (fertility, bereavement, illness), SAHMs, Dads, knitting, and more."
Across the country, people use Facebook groups and listserves to create local peer communities to support one another.
Although outdated, the listserve's functions (a least the one I use) meet most of the needs of expecting parents and parents. With that said, there is a lot of room for improvement. Here are a couple of pros and cons from my point of view - Pro: they are hyper local and can have vast resources within the group. Con: Listserves have poor UX - the info is communicated by email - and have no centralized resources for parents unless the group has a website.
These groups would be a great resource for further research and prototyping.