You could look into
Quip which is used for handling blog comments. I haven’t used it before but there should be a way to get it working with AJAX.
If that doesn’t offer the functionality you require I think you’ll have to look into creating a custom schema and database table to handle your content, have a look at this tutorial here:
http://rtfm.modx.com/display/revolution20/Using+Custom+Database+Tables+in+your+3rd+Party+Components
On your ’twitter style’ page you’ll need a custom snippet to loop through and display all the relevant existing entries from your custom table. This can be very simple but you’ll need to familiarise yourself with xPDO (see tutorial above).
You’ll need another snippet to handle your AJAX request to save the new post to your table, and return the new content to be appended to the existing page content. You could look into
Formit to handle the form data.
You’ll need a bit of patience if diving straight into some custom coding, however there is plenty of help here on the forums if you need it.
ps. Just noticed your last comment... In what ways did you feel the forums are lacking? Your first post might have slipped through but if you stick around you’ll find this is one of the most helpful communities out there.