Last week at the "Voice of young scientists" media workshop, several early career scientists expressed an interest in communicating via new or social media but weren't sure how to get started (http://www.senseaboutscience.org/pages/voys.html).
Here are a few useful how-to articles to get you started communicating via a blog or twitter.
These posts will answer most of your questions, including how and why:
(The "blogroll" of the first two posts lists links to many of the top science blogs to inspire you).
You can start your own blog on wordpress or blogspot or other hosting sites (I like Scoopit). If you want to write a single article rather than start a blog, you can try to submit a guest post to an established blog like those in the blogroll. Science blogging is competitive though, so may be hard to get a high profile guest post without having an established blog already. Alternatively, look for a lower profile site, such as might be provided by your university, or a discipline-specific site. Fledgling group sites are often looking for contributions - here's one actively seeking content http://www.thetwentyfirstfloor.com/?page_id=2995. Or, start a group blog with colleagues!
Twitter has been embraced by a lot of leading scientists and science communicators. Its free and safe and you can be completely silent but still get a lot out of it. Start by creating an account and following people and organizations for a bit before you jump in. Most of the writers for influential blogs tweet, and you can find their user names on their blogs. I also follow my favorite journals (including @sciencemagazine and @naturemagazine) for updates on new content. Retweet things you like, and tweet links to content you find interesting, including your own blog posts!
Most questions will be answered here: http://socialnetworkingforscientists.wikispaces.com/Twitter or here http://bit.ly/RQS7Ip
Here's a first person account from @biochembelle (http://www.asbmb.org/asbmbtoday/asbmbtoday_article.aspx?id=17900)
"Below the line" commenting
You can also be a very effective science communicator by responding to newspaper articles about science "below the line" (in the comments section). For me this is the hardest, and I don't do it much, but I appreciate people who do. Most articles about nutrition, climate change, GM plants etc generate reams of comments from the scientifically ignorant, and I applaud those who take the time to point out errors, post links etc.
Here is an example from Slate in which "MThomas" has corrected some of the errors made by commentors (http://slate.me/TDYSLU). Even if you aren't an expert, you can respond to many common misbeliefs by pointing to the excellent "Sense about science" resources (http://www.senseaboutscience.org/resources.php).
Science needs defenders, and scientists need to contribute to the virtual discussions about science taking place 24/7, so go ahead and add your voice.
[Image by Paola peralta (http://en.wikipedia.org/wiki/File:Social_Media_Marketing.jpg)]