Nextdoor is a semi-closed neighborhood forum for talking with people in your neighborhood.  Learn about lost cats, stolen porch plants, local grants, upcoming neighborhood development concerns, and more.  Portland has an active Nextdoor community with many members.  Any given "neighborhood" may have a thousand or so people you can interact with.

Nextdoor Communities in Portland

See also - similar to Nextdoor, but smaller, less conservative and with more of a "pay it forward" focus.