Dear Mr. Zuckerberg,

What a ride! Your college prank site has morphed into a global phenomenon with over 900 million users world-wide. Facebook ran one third of all banner advertising on the Internet last year. That’s incredible market share. Your site is so ingrained in the daily lives of so many people, it hardly seems possible that it’s only 8 years old. Gosh, they grow up so fast these days, don’t they?

But I think it’s safe to say, the honeymoon is over.

Now that you’ve decided to bring real money into the equation, people are finally paying attention to what they’re doing with their time and how Facebook makes them feel about themselves. Look, I understand that 30,000 servers cost a lot of coin, and that you and your investors have a right to earn whatever you can from an IPO. Facebook is not a cure for cancer, but, hey, neither is Coke or McDonald’s. In fact, you can argue that Facebook has done more for social harmony than 900 million Happy Meals ever could. But now that you’ve begun to cash in a few of your chips amid enormous fanfare and an investment bank screw up, people are asking questions.

My favorite was the AP/CNBC survey that found 46% of Americans think Facebook is a passing fad. The remaining 54% must have been too busy updating their status to take the survey. LOL, I’m kidding! In reality, 43% believe that Facebook will be around for the long haul. They were probably the first ones to overpay for your stock when your IPO hit the ground stumbling and fell on its, um, Facebook.

But a far more ominous tone was sounded by GM, which, just days before your IPO launched, made a big splash by announcing they were pulling $40 million in advertising from your site because it “wasn’t effective.” Oh, snap, Zuck, did you do something to tick them off? Their move was about as snarky as your original site concept which compared Harvard co-eds to each other in a mean-spirited head shot contest. GM made a big deal out this when they could have said nothing. Of course, that happens 50 times a nanosecond on your site, doesn’t it?

The issue, of course, is the money. I’ve said before that Facebook has the potential to be the biggest media vehicle ever. The personal information that we slather all over our profiles is a marketer’s dream. You can use your site to deliver an unlimited amount of content-movies, games, music-in exchange for exposing ourselves to highly targeted advertising.

But as a brand, you are walking a fine line, Zuck. As long as we feel like we are gaining from this relationship, we’ll be OK with how you sell us to the highest bidder. But the minute we feel exploited, you could saddle your brand with a stigma that will be hard to shake. It won’t be long before estimates of revenue-per-user start to filter into the mainstream media. When they do, get ready for a lot of questions about exactly what you are doing with our information.

I’m not ready to side with the 46% folks who think you’re a fad. But as a brand, I wonder if maybe you’re close to your peak. Your revenue actually went down in the first quarter of this year, which isn’t helping your stock price. Any brand with 900 million users must be doing something right, even if very few actually give you any money. You were smart enough to realize that we’re giving you something just as valuable-our attention and our personal information. You’re the media vehicle, but we provide the content. (Imagine if TV networks could get away with that.)

So, Zuck, Facebook is at a crossroads, IMO. Just as usage is showing signs of leveling off, your new stockholders are looking for their investment to go up in value. The pressure will be intense to deliver ad revenue while keeping the Facebook experience fresh and competitive against challenger brands. Welcome to the world of profit and loss. You’ve done an amazing job of building the Facebook brand. Now let’s see if you can sustain it. (PS: Click here if you “like” this letter. LOL.)