Since .threads uses MySQL for data storage it's ability to grow is only limited by this and the hardware that it runs on.
Here is some data on MySQL.
Scalability and Limits:
Handles large databases. We are using MySQL Server with some databases that contain 50 million records and we know of users that use MySQL Server with 60,000 tables and about 5,000,000,000 rows.
I have seen a .threads site that had over 170 users listed in the "Who's Online" page. For the life of me I can't remember what site that was. I offered to help someone and they gave me their URL and I can't even remember if I actually was of any help either... lol
Having plenty of RAM and fast drives such as the SCSI types will improve performance as far as hardware goes. Fine tuning MySQL and having PHP compiled into the web server helps also.
Yahoo! recently decided to go with PHP for their own uses and I believe that also included MySQL as well. (this in itself says alot to me as far as the potential of .threads)
