As for not jarring people when you introduce them later: 99.999% of your visitors will not arrive on launch day, or in any short window from the opening of your service. From the perspective of the guy who signs up on your SNS twelve weeks from now after reading about it on Bob's SNS Addict Blog, the site had ads from launch day. But if you actually put ads on it on Launch Day, you might not get that link from Bob in the first place.
P.S. On the other hand, if you charge money, charge money from day one. Even if you're not actually charging anyone because you mailed Bob a free invite code because you dig his blog especially that series on MySpace monetization and thought your new site would interest him. Free stuff = meh, the Internet is full of it. I am getting for free what other people are paying money for = proves my status as an expert on this field (validating my self-worth) and makes me like you enough to want to write a review in which I will (strike)brag(/strike) disclose that you thought I was an expert worthy of a freebie.
Edited to add: SNS means "social networking service". Apparently its a Japan thing. Sorry!
Why not reserve some space for ads here and there from day one, but fill them with ad-looking text (à la google ads) about this or that feature of your own app? Then, a month later, replace those ads with real ones.
Those should definitely not be involved in any workflow of the service.
My point is just to soften the transition, by preparing some ads areas so that they don't appear one day, stealing space from the app.