As far as Active Rain, or any other blog; search engines look for new and relative content. If you build a site and let it sit, there will be nothing for search engine spiders to look at that is "new and relevant". Let's say your site is about homes in Chesapeake Va and you blog about Chesapeake Va, you can now link your blog, to your site with a relevant phrase "search for homes in Chesapeake Va" you now have a link, which is what search engine spiders love that ties together new and relevant. The more you do this and link the two, the more search engine spiders will pick up on it. So when someone Google's "homes in Chesapeake Va" you're going to have multiple links with relative and new information that will index you to the top.
That's about the best I can do for an explanation. It's free, it's you, and it helps you to connect with what you have a passion about. I know that when I started If I Googled anything about any of the area's I work I would be lucky to come up on page 5 or 6 maybe, now I come up on the first page, usually in the first couple of spots.
It's not a magic button and it takes some time, but the more you do the better you get, and the best part is you learn more about the community you serve which makes you more valuable to your clients.
As far as Plaxo, Twitter, Facebook, Myspace, and the other 500 sites out there, it helps me to connect with new people and old friends that I have lost over time. You can joke, laugh, talk about the old times, but most importantly if they know of someone looking to do business in your area, hopefully they will think of and recommend you. Of course it doesn't compare with getting out and spending time face to face with your past clients and friends. Although when its late at night and cold out, the kids are in bed and you're in your PJ's, it's the next best thing.
The leads come from people finding you on Google searches, and from people interested in what you have to say about the community they live in.
I too am a beginner but I'm learning, I hope this helps, and I hope you have a great year!
Brady Howard

Great blog. I also don't understand why people give up on their blogs and let them sit. I'm still learning Twitter and Facebook as I just joined both a couple of weeks ago, but I would never give up AR as it makes me too much money!