Database structure of Facebook to store comments and post?

Small example of what you could do:

Table users:

users
--------
id
name

Table comments:

comments
--------
id
comment_text
posted_by
posted_to
date_posted

When user with ID 342 posts a comment to user 783's profile, you can store the comment like so:

INSERT INTO comments(comment_text, posted_by, posted_to, date_posted) 
VALUES ('Hi!', 342, 783, '2013-10-16 11:17:00');

Then, when you're selecting comments to show on a user's profile, simply join the posted_by and posted_to columns up with the relevant ids in the users table.