My start-up will be similar to Doostang and LinkedIn and will have some elements similar to those of Facebook. To design such a site, I am debating whether or not to use a CMS. I have checked out Drupal and Typo3 and they seem like they would work for what I'm going for. I would like to get the HN community's opinion on whether using a CMS for such a project is smart or not. If not, why? If so, which CMS would serve my needs?
I wouldn't. When you are using a CMS, generally speaking you will be limited in functionality due to the CMS being your framework. When you code from scratch or with an established framework (Symfony, CodeIgniter/Kohana, for PHP) you'll have a lot more freedom. People have had great success using Drupal as a backbone for an application, but I can't suggest it, since you're building upon a CMS, and things can get tricky that way.<p>With that said, you mentioned TYPO3 which has its own PHP Application Framework, FLOW3: <a href="http://flow3.typo3.org/" rel="nofollow">http://flow3.typo3.org/</a>