If you want to start a blog, start one about something you're interested in. Not only will you learn everything about social media and blogging that you might learn with a PR blog, but you'll also learn more about something else. While I don't doubt that you do care about PR/Communications/Social Media, I really like to think that you've got other interests (if not, then your problems are greater than my ability to help you with them).
If you're young and think you want to get your start in PR or communications, that's great. Prospective employers will be happy to see that you have a blog and are involved in the online space. They'll probably be happier still to see that you've got enough originality to think outside this realm, and become an expert in something that interests you. You'll also be able to demonstrate that you understand the other side of the media fence.
You can still be involved in the world-changing discussions about social media and the future of PR that happen all over the web these days by reading and commenting. In fact, by blogging outside the social media bubble you might even become more of an expert than some of those social media consultants and gurus.
UPDATE: Darren Barefoot wrote a similar post today called Writing About What You Know. Definitely worth a read.