The short answer: I think, you care too much about the wrong question.
Now, the long answer: Promoting something as "free" certainly is a powerful tool. This is because any of the two reasons apply:
- The offer removes monetary costs (ie. what people think it costs).
- The offer removes cognitive costs (ie. the problem to make a decision).
But many people have certain rules of thumb for decision making which run counter to a free offer. Examples are: "You get what you pay for!", "If it sounds too good to be true, it usually is!", etc. A free offer contradicts these rules of thumb, thus it will create cognitive dissonance.
To prevent cognitive dissonance, people change the one piece in the equation that is most easy to change: the evaluation of your product. They either devalue your product because it's free. Or they revalue your product because it costs more than "usual".
Just recently, some statistics about iPhone app market showed these two effects rather clearly: Higher Price, Better Ratings. Don’t Discount Your App At Launch.
However, this does not necessarily apply!
For people also use context information when making judgements. Think promotional give-aways in super markets: Most people accept these free offers without devaluating the products. Everybody knows there's a reason for these free offers. Nobody's asks "Where's the catch?"
So, what are relevant context informations? Some examples:
- The supplier has an incentive to make a free offer.
- The offer is restricted (either in time or space or to a certain group of people).
- It's a web site.
The last one currently applies to you. Let's face it: People are used to use web sites for free. It's not a big deal. They are also used to get SaaS updates for free. It's not a big deal, either.
So, the chances that your product is devaluated is rather low. At the same time, the chances that the promotion will create a sort of Slashdot effect, is low, too. Of course, it's also a matter of presentation: If your copy screams "FREE! FREE! FREE!", people may still get suspicious.
So, I'd say, promote these new features as best as you can. Mention 'free' if you'd like to but I wouldn't if I were you.
For if we're talking about the office thingy you linked in your profile, you have other problems. You need to solve the chick-egg problem, because that's probably the more important objection of your target audience: Is there sufficient demand/supply or will I just waste my time?
Your page may be one of the thousand fun pages created by bored web designers. That would waste my time. But nothing indicates, you're not! The little dog image is easy to overlook during a quick scan, for it looks like a company logo. There's no pricing page, so you don't seem to be interested in making money. Thus, you're also unlikely to promote the page appropriately.
If you'd promote your new features as free, you may sound desperate. I would want to prevent such impression at all costs if I were you.
Additionally, your FAQ says there's no fee for posting a space, but it says nothing about fees for searching or finding a space. That's highly suspicious! Taken all together people may really wonder, where's the catch? But not because of your new feature roll-out.
Hope this helps.