Search the Members list or click on a members avatar or name to open up their member card. Click the Messages: NUMBER and it will display all the posts and threads the user has posted. Another way is to click on your name at the top of the Right Sidebar. It will drop down all of your settings. Select "My Content" to bring you to your profile page content. Click on Profile Page then the Postings tab to get the same listing of all the posts and threads the user has posted. At the bottom of the listing you are able to sort threads or posts by user.