Will depend on how much time/page loads the average user has. Social networking tends to be "sticky" according to Facebook, so people hang around for a while. More page loads means more load on the system. The code behind the site will have another huge effect, better code will put a lighter load on the system.
These days if you don't have a good idea how much/how fast your site will grow you might want to consider one of the cloud hosting environments like EC2 or Rackspace Cloud/Slicehost. You can buy two server instances to get started, and add more servers quickly as load changes. Experience with your app is the best way to get a solid idea on how much capacity you will really need. Excess capacity sitting around is expensive, so avoid it if you can.
Having said that, 100,000 users isn't a huge load if they only load the page a few times a day. You should be able to get started on that with as little as a single server and probably no more than 2-3 total.