If I were to start from scratch using a free blogging platform, I would go with tumblr. I don't know any other platform or social network where things get liked and reblogged (reshare) like on tumblr. Things go viral pretty easy on tumblr than any other platform online. I think it's because there are no rules, anything goes on tumblr D:
No need to stress too much about traffic, you can get traffic within the site and whenever you engage, reblog, like, follow people and post compelling content, you'll attract a lot of eyeballs much easier as compared to popular social networks such as facebook and twitter.
You can go ahead and use other platforms such as wordpress and blogger but make sure you have a good marketing plan because from my experience it's really easy to make a tumblr blog popular. Tumblr has a really great community, it is both a social and blogging platform.
Even if you check the stats for websites that get the most active users each month, tumblr is right below twitter and google+. So tumblr get truck loads of traffic!
You can easily move your blog to a self hosted one later if you want to, same with blogspot as well.
1. It is a wicked blogging platform - easy to use
2. If you want to later move from free to a paid hosting, most reputed hosting companies comes with a one click installer for WordPress.
3. Custom build themes and ability to manage multiple free blogs using single account.