Entrecard is a social blog advertising system with a whole lot of potential and a number of pitfalls which, with a bit of experimenting, can be overcome.
For those who have never understood the widget, here’s the deal: you place a widget with a 125X125 advertising space on your blog. Below it is a little Entrecard line that says “Drop”. When other logged in Entrecard users surf by, they can “Drop” their card on you. Their blog then appears in your Entrecard dashboard and you can (if you wish) reciprocate the drop by visiting their blog and dropping your card. For each time you drop, and for each time someone drops on you, you get one point.
With those points, you can buy advertising on other people’s site widgets. And here’s where it gets tricky. The more people ‘drop’ on your widget, the more credits it will cost to advertise on your site. However, many of the sites that have the highest advertising rates do not have the highest quality traffic. If, for example, the blog you’re advertising on primarily gets its traffic from people reciprocating drops, they’re unlikely to click through the advertising widget to your site. And often, if power droppers do click through, it’s only to drop their card and move along.
While I’m hardly an Entrecard guru, I have picked up a few things from playing with the system for a few weeks.
So, my first tip is this: Don’t be in a rush to advertise with the highest priced sites in your niche. First, check out their profile. If it’s going to cost you 500+ points to advertise on a blog, and the profile says the blogger is:
Deeply committed - Deadly serious. A drop master
That’s your first clue that maybe the traffic isn’t going to be quite what you’re hoping for. It’s a clue, mind you, not a bang on certainty.
To find out more, head over to the blog and see how many of the recent posts are about Entrecard. If most of them are, you’re again looking at a pretty good clue that nearly all the traffic coming to that blog is from one source. (And you’ll never guess what that source is!) 
Give the blog a pass if the ad rate is high and the blog hasn’t had a post in a week or two. Definitely.
Finally, check the post comments. Make sure there’s a fairly good variety of people posting, rather than the same few on every post.
If the signals are mixed, flip a coin when deciding to advertise. After all, credits come cheap, and at some point it gets hard to dispose of them all.
Now that I’ve pointed out a few of the signs of a bad purchase, here’s a must-buy in any niche… find the bloggers with ad prices over 40 credits who are given the drop rank of:
Just here for the view
or
Casual and relaxed
You then have a good idea that they get traffic from places OTHER THAN Entrecard. Its no guarantee, but it’s a good place to start.
Some of the best traffic I’ve gotten through EC is from a few quality blogs that hover between 20 EC and 80 EC to advertise on. Apparently I’m not the only one finding out these guys drive great traffic, since it’s getting harder to snag advertising spots in their queue.
I, apparently, am “A dedicated dropper with a hint of addiction.” To achieve this lofty title, I basically reciprocate my drops inbox every day, and surf my way through some of the more interesting ads on weekends.
To reciprocate without pain, I use the Firefox addon “Linky” to quickly load up all the sites in my drop inbox in different tabs. (To do this, install the plugin, go to your drops inbox, highlight all the ad boxes, then right click and choose “Linky-> Open Selected Links in Tabs”. Then go grab a coffee while all the sites load, then whip through the tabs dropping your card on each of those blogs and closing the tab when you’re done.) Using this method I get through about 50 by the time I’m done my first cup of coffee for the day.
Finally, when you’re through with all the productive things you should have done in any given day, or while you’re in the middle of a really boring conference call, you can use Sitehopping to autosurf your way through Entrecard sites effortlessly.
