The quick way to send someone a PM is by clicking on their avatar (or the circle where it says "no image") from any of their posts. This brings up the user's mini profile from where a PM can be sent using the speech bubble icon in the bottom right.
The @ icon brings up the email form and there maybe others depending on the user's profile.
Clicking their username itself from any post, or the View full profile
button from their mini profile shows their main profile page where the same icon is available along with other options such as searching for all their posts.