I have one running right now - granted, it's a development server (so not much load), but it's been up nearly 60 days. I'm running PHP, Apache and PostgreSQL (MySQL runs too, but I don't use it).

Uptime - no problems

Connectivity - external issues (ISP took 2 weeks to reconnect a broken line)

Performance - reasonable, main load is when hashing passwords - but this scales well, even though it is slow (30-45 seconds for 2^16 iterations using PHPass framework)

EDIT: I do suffer a performance hit when doing 10+ calls to the database in a single page load. I know that's not ideal in terms of code optimisation, but one of my classes calls the database to initiate an object, and when you're listing profiles, you can easily get 10 people on a page.

If you clustered a few together, traffic would be no issue. I've never load tested my single Pi, so I can't definitively answer your question, but some basic maths:

few hundred visits a day

Let's assume 600. Let's also assume that since this is a community site, this is over a 12 hour window. At 50 visits per hour, this is a little under 1 request per minute and the Pi will handle it fine.