Ask YN: Recommendation for a social network platform?

My company is getting prepared to rebuild our existing social network from scratch. Our current network (niche, B2B site) is "ancient" -- we built in 2003, on ASP/MSSQL which is what we were running on then.

I have investigated a number of options, and I'm not 100% satisfied with any. If anyone has thoughts, experience or recommendations I would to hear it. Open source is our preference by the way.

My quick notes on what I've seen so far:

* Elgg. Open source, looks cool. However, they are rebuilding the core from the ground up for 1.0? And there is no codebase or deadline to speak of...just occasional blog posts from their core dev team. Anyone know anything about it?

* Buddypress. Promising, but in development. I haven't also figured out if the MU blogging platform is a square peg they are trying to fit in the round hole of social networking?

* Insoshi. It's been posted here, I think it's YC?

* LovdByLess. I get the sense it's a social network that you can have the code for and basically customize, rather than a platform from which you can create a social network and extend it.

* Social circle. Nice guys, and looks interesting / well designed. Powers the social network acts_as_community which was posted here on YN last week. Not open source, and our company can't really afford the prices I'm afraid.

Of all of these, Elgg looks the most promising...but there's a giant question mark for me without knowing more about the development.

Last option to consider is building our own, from scratch, on a modern framework like Django or Rails.

Well, I'd love to hear feedback if anybody has any. You guys are great.