Django or Ruby on Rails
I just finished building a small social networking site with Django for a client.  I was new to Python and new to Django, but it was one of the most enjoyable development experiences I've had in a while (even with the headaches of being new to a language and framework).  And it only took two weeks.
If you're building this for yourself then I would try both Rails and Django (or one of the other frameworks that c.batt mentioned) to see what feels the most comfortable.  I ended up choosing Django/Python because I liked the language.  It felt clean and efficient, and you can't argue with the speed at which you can get a bare-bones site up and running.
As mentioned previously, Pinax is great. If anything you get a solid base to build on top of.  I found that I had to extend parts of it to get exactly what I wanted, and a couple of sections I decided to code from scratch because the changes would get "hacky" to match what I needed. Other parts, however, are perfect.  The private messaging was used out of the box, just with some new templates to match the site.
I'm a C#/.NET developer looking to mess around with something completely different - something LAM(*) stackish for building web apps quickly.
I'm thinking either Django or Rails. I kind of like the Python language better and it seems to be more full-featured than Ruby for statistical, scientific and networking (let me know if you think this is wrong). However, the RoR community seems to be much bigger - which might make it a safer bet. Do you think this is important? Anybody have experience with both RoR and Django?

Update: Specifically I'm looking to be able to build a community finance oriented site quickly. There are definitely a lot of Ruby Gems to speed up the process, but I noticed Pinax on the Django side which looks promising. 
I know that Python is already fairly popular for financial/mathematical programming. Anyone else have an opinion?

Update 2: Noticed some comments about ASP.NET MVC. I have in fact done pretty significant work with ASP.NET MVC - a LOB app for Medical Equipment Servicing, and I loved it. It is IMHO a much better and more intuitive way to write web apps compared with ASP.NET web forms. But, I'm really looking for a non-.NET development environment to develop some chops in.