The users_sessions table is where Elgg stores the PHP session information.  If you delete it you will log out all the users on your site.  There should be garbage collection to clean it periodically, but I don't see that in place...

garbage collection does not clean it 

this table is growing fast on my install elgg 1.6.1

more than 1490 records in one day and only two users have logged in the site

This is where PHP sessions are stored, so any unique user to your site will generate a new session.  Elgg tells PHP to store session information in the database, but if it didn't, PHP would have created 1490 new files for them.  This isn't a problem.

What is a problem is how PHP's native garbage collection deals with custom session storage--I have a hunch that it doesn't.  More research will have to be done on this, and I'd imagine a garbage collection of our own will need to take place for the sessions to keep this table in check.

The PHP setup that comes with Ubuntu (and Debian?) is broken for custom php session storage. I believe with a proper configuration, session garbage collection works but I have never checked.

One more thing, do not fix the php settings for garbage collection if you have a ton of old sessions that haven't been cleaned out. That'll probably cause mysql to go bonkers.

i clean it by hand , thank you :)