I’d also be interested to know how scalable BuddyPress is…? Never really tested it past a few thousand members.
The BuddyPress team should be able to give you a better answer, but one thing I came across while deciding about whether to use BP or not was that in the UK the Daily Telegraph’s user section is based on a custom version of BuddyPress and, a year or so ago, was reported to have around 150,000 users, which at the time was thought to make it the biggest site in terms of active users.
Since then BuddyPress 1.7 re-engineered queries to make them much faster when working with large DBs.
I would have thought that if you get to a position where you are pushing the boundaries of what BuddyPress (and WordPress) are capable of, you should have the resources to invest in optimisation/customisation as required.
All this assuming, of course, you are doing everything else possible to improve performance and are not running your mega-site on some cheap shared hosting plan.
Yes, I think one of the BP core developers worked at The Telegraph before his move to Automattic.
I would guess the number of active users should be the main metric to use. A site with 1m inactive users will test just how much data the server can hold whereas a site with 1m active users will test so much more.
See this response I found on Quora from @jjj: