Create social network in Joomla--issues with migrating to Drupal later on?

Personally if you're definitely leaning toward moving to Drupal later, I'd say go with it from the start... while you would be able to transition Joomla's users and primary content to Drupal fairly easily (I believe there are some conversion scripts for that), it's unlikely a lot of your additional content will make the transition very well unless you write your own conversion script and/or mysql queries for the task (e.g. it's been a while since I used - and actually started out with - Joomla, but I believe comments and many community-focused features are all 3rd party... and any stock conversion script will not be likely to help you out with any of those beyond what core Joomla does out of the box). Drupal does a lot more of the community features out of the box, and the contributed modules you'd need are more often than not highly used and supported.

I started out with Joomla 1.0.x a few years ago and grew to dislike it strongly (to get it to do what I really wanted always ended up requiring me to hack away at core or extensions files, making upgrading a nightmare)... though I've not tried it extensively I believe 1.5+ is much better than it used to be. Though I still feel Drupal comes out on top if you want to create exactly the site you envision in your mind. Once you learn the key tools in Drupal (CCK and Views for the most part, seasoned with a variety of other key modules, many of which extend those two "parent" modules in a plethora of ways), you can make pretty much anything you can imagine in Drupal, and do so with little to no compromise to your vision. You can achieve this also with little to no PHP and database understanding as well, since CCK and Views pretty much do all that for you, leaving you with just theming to do (very very basic PHP, you'll pick it up quick) plus the normal HTML and CSS. If you need PHP beyond that, it's almost always only because you want to override how Drupal core or a module does something, and do it your way instead... which by the way is part of what makes Drupal better (the fact that you "can" do that, without ever ever ever modifying the original core/module code, so upgrades are always easy). For instance I'm making a community website myself where I've wrote a custom module or two to do what I want, but that's only because I'm being super picky about making every single detail the exact way I want it... I "could" easily use a few contributed modules such as Advanced Profile Kit, Content Profile, Panels, Advanced Forum, etc, and avoid most or all of my custom coding.

If you decide to go for Drupal, focus on learning CCK and Views, understanding what Taxonomy can do, learn about theming, and familiarizing yourself with the most popular/recommended modules (for instance, look at the Community Modules that are used in the Acquia Drupal distribution, all of which are highly stable and widely used by countless sites... I also link to many good ones in one of my own site's case studies... and check out the drupal.org case studies). There's mountains of learning material, though if you want as fast a start as possible, a book or two such as Using Drupal or others would help you greatly. Also check out http://drupalsn.com where you can get ideas and ask community-site specific questions. Also stop by the Drupal IRC channel for real time help with your questions.

Best of luck :)