Patrick Foley did a great job at describing the advertising advantages of using Craigslist. So rather than re-list them, I suggest visiting his post. In fact, we're already using it for promotion of the upcoming launch of our Sibdu platform.
There's two others, though, that can work great if used properly. One you'll only use if you're trying to attract other real-estate pros (for referrals, etc.), while the other you can use to target consumers and B2B (business-to-business) who are interested in either commercial real estate or investment real estate.
The first is right here at good ol' ActiveRain. Since you won't use this for consumers (that's what Localism is for), you'd use this to get referrals, or for other reasons I can't think of right now (but it's cheap, as you'll see below). People like me, meantime, use AR to attract new customers, so we use it for impressions, sign-ups, direct sales, brand building, etc.
ActiveRain uses CPM (cost per impression) as its model. This means you pay for each "impression" that an ad makes (per one thousand impressions); in other words, each time it appears on a page. Now this can get expensive, depending on the site. But here at AR, we're getting CPMs anywhere from $0.68 to $3.69 per thousand, so this is pretty darn cheap. We've got $60 scheduled for this campaign, and we're going to be getting thousands upon thousands of impressions for this amounth of money.
The second ad vehicle is Google AdWords. I know, I know, there's been a lot of derisive comments out there about AdWords. My opinion, though, is that you have to know how to use it to use it well. There's lots of books, etc., out there that can help you use it effectively. Perhaps the best, though, is the help Google provides right at the AdWords site.
Unlike ActiveRain, Google uses CPC (cost-per-click, also known as pay-per-click), which means you pay for each person who actively clicks through to your site. Personally, I'm not sure which is better -- CPM or CPC. Both can and do work, though.
We're now using all three to promote the upcoming launch of Sibdu (Commercial Real Estate Property Search by Request). I just started Craigslist and AR yesterday, and rolled out AdWords today. Just based on the first two from yesterday, we've seen sign-ups increase by at least 200%. I can't wait to see what happens when AdWords gets fully rolling for us. I'll update you then.
UPDATED 1:05 PM, in re: to Bob Stewart's comment below.