Mathew Ingram has an interesting post on GigaOm today about why Facebook should buy Tumblr. He lays out a compelling case of Tumblr’s insane growth, which I admit was bigger than I’d expected.

As I read I nodded my head, until I stopped reading, thought about it, and suddenly realized that buying Tumblr is the absolute last thing Facebook needs right now.

First, let’s look at Ingram’s reasoning. A deal for Tumblr is certainly not out of the realm of possibility. I would be surprised if Tumblr ever became a stand alone public company. And clearly, Facebook has advantages in getting a deal done with Tumblr that others may lack.

I doubt a Yahoo — or any company that is truly desperate for Tumblr’s hip demographic and gargantuan traffic– can get this deal done unless the money is just insane. Tumblr has a position of strength, and it’s founders have already taken a good deal of money off the table through secondary rounds. There’s no immediate pressure that I know of, and I’d be stunned if Tumblr founder David Karp wanted to be some has-been Web 1.0 company’s life raft.

And Facebook — unlike Twitter — has a publicly traded currency, which always makes acquisitions easier. Karp would actually know how much he’s selling his company for, unlike Kevin Systrom who got about $300 million less for Instagram than he’d thought by selling to a pre-IPO Facebook.

Ingram’s weakest point is the this-is-how-my-teens-use-it argument; always a tempting argument but one that should usually be resisted. This was the logic that lead VCs to pump more money into WebVan than any other ecommerce company save Amazon. But our wives want to pay more money not to have to go to the store! Not exactly how the average middle American housewife thinks. If we’re going by that reasoning Facebook should also buy Yo Gabba Gabba. My son can’t get enough of it.

More to the point, I’ve heard just as many people say their teens are so fickle that they rotate between these services regularly making any real behavioral trend hard to pinpoint. The key for any lasting social network seems to be not being reliant on this demographic at all for success. (Amiright, Formspring?)

But here are the bigger issues with his argument:

1. Tumblr is not another Instagram. This isn’t an issue of size. What scared Facebook about Instagram was that it was all about photos, and it was all about mobile. The former has always been Facebook’s core strength, the latter was Facebook’s biggest weakness. Tumblr simply doesn’t share those traits. It’s primarily a Web property, like Facebook, and is more about sharing videos and gifs and short micro-posts than sharing photos with your friends. You can’t just point to two surging social companies that are still private and say they represent the same thing.

2. Facebook should not be acting defensively. Yes, I know Wall Street has it in for Facebook, and morale has been tough at One Hacker Way. But Facebook is a still-growing, newly-public company that has done something no one else in the world has ever done: Wired 1 billion people together through real world relationships. We’re not even at the beginning of seeing what that means for the company. The deal Ingram describes and the fear that he says Facebook should feel from Tumblr is the way you describe a Yahoo or even, maybe, a Google. But not a Facebook.

Dear God, if Facebook is thinking this way the company is in far worse shape than any of us know. If it’s already at the point where it’s going to go around the private Web Hoovering up any and all companies that might be a threat, things are going to get even uglier for the company. A string of mega acquisitions are distracting in practice and by design– they are a way of masking organic growth numbers from Wall Street. Facebook needs to focus now. If the company does buy Tumblr, sell the stock. Because that’s the move of a scared company.

3. What the hell is Facebook going to do with a third social network? I find it hard enough to believe that Facebook is going to keep Instagram separate. There is no way on earth this company — with all of its challenges — needs to be running three huge, separate properties. The waste of resources, distraction, silos, turf wars and infighting would be tremendous. When we asked Systrom whether he could assure users that Facebook and Instagram would always remain distinct he said he couldn’t assure anyone of anything that far into the future.

And, again, for the reasons outline above, Instagram was a rare exception to Facebook’s typical M&A playbook which is about buy small, kill the product, take the team. Tumblr would make it a pattern, which would be a rapid departure for Facebook and a signal that something is not right deep within the company.

4. More ad pages. Oh, hooray. There is one clear thing that Tumblr could bring to Facebook immediately– a shitload of ad inventory. And no doubt that would please Wall Street temporarily. Again, this would be a textbook move if Facebook were a declining Web giant like Yahoo, desperately trying to stave off activist shareholders. Just keep bolting eyeballs on because there’s no next product revelation coming. The only hope to grow the business is volume.

But that’s not the position Facebook is in.

As I wrote before, what Facebook needs to get out of this funk isn’t a billion more users or 20 billion more page views a month. It needs a better way of monetizing its users and the unique role that it plays in their real-world lives.

Facebook CEO Mark Zuckerberg isn’t stupid. He’s spent much of his time as a public company CEO messaging that Facebook will become less reliant on brute force display ads, not more reliant on them: Whether that’s getting into search, some sort of ad network, native ads, or even gifts.

If that’s the only rationale for the deal, it’s a pretty weak one.