It means there is another Forum with a nearly identical name, which was founded by a former member of SSF who left in huff, and several others started by former members who weren't happy with the way this Forum works. This Forum has more members than all the imitators combined. So, don't be satisfied with a pale imitation of Slingshot Forum, stay with the original and best.
*This is my personal opinion, based on experience and observation, and does not necessarily reflect the opinion of management.