No matter how useful an interface may be for a website or mobile app, it will grow stale after a couple of years. New features will appear. Your rivals will cherry-pick your best ideas and fold in their own. A few years of stagnation are all it takes to become an antiquated joke.

Facebook (FB) understands this as well as anyone. It’s retooled its profile page and news feed several times in the past nine years. Each time, the changes have been followed by an angry wave of discontent, then a calmer period of acceptance. The redesign this week will be no exception. Some are arguing there’s plenty to hate in the new news feed even before it’s been rolled out to everybody.

Based on the presentation Thursday by Mark Zuckerberg and others, the revamped news feed is appealing enough, with a more spacious layout for photos and videos and the ability to move easily between different feeds for different content rivers. But if you look closely enough, the redesign also reveals something else Facebook may not be happy talking about: the vulnerabilities the company is facing as it moves forward.

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The first vulnerability is that the dominance Facebook has in the world of social networking has slowly been eroding. Beyond the usual talk of Facebook fatigue, newer social sites have risen up in recent years to steal attention away from Facebook: Pinterest, Tumblr, even Google+.

So as Facebook unveiled its new look, many of the comments on different forums echoed each other: The new photo-friendly layout drew a lot on Pinterest. The river of big photo and video feeds looked awfully like Tumblr. Most of all, it seemed, the new Facebook news feed had a Google+ feel to it.

If imitation is flattery, Larry Page must be smiling. But while it’s common enough for competitors to pluck design ideas from each other, it’s not a positive sign for Facebook. It’s simply not emblematic of a market leader to launch a new redesign of your product comprised largely, if not wholly, from ideas that originated inside other companies.

More and more, Google (GOOG) is proving a stubborn rival to Facebook. Not just Google+ or even Facebook’s acknowledgement that search was, after all, important to the company. Google has simply done a better job of keeping its core product simple. Google.com is the rare exception to the rule of redesign or die. For more than a decade, it’s evolved only in subtle ways. Yet it’s as useful as it ever was.

This gets at a second vulnerability facing Facebook. The more the company tweaks the news feed — the very heart of the Facebook experience — the more complex it becomes. Is the news feed a chronicle of friends’ activities? Or news stories? Or music and video clips? Which photos belong at the top — those taken by friends or those shared Pinterest-like? Some users demand a reverse chronological order, others prefer Facebook’s algorithms.

Contrast that to Google’s little white search box. Type in a few words and the algorithms do the rest. Of course, algorithms are so much better at powering search engines than they are anticipating the content discovery that fills Facebook’s feeds. So Facebook has responded by letting users switch between multiple feeds, customizing new ones, tweaking their preferences. It adds up to more work than most people are too busy to bother with, and it shows how unruly Facebook has grown over the years. Twitter’s simple reverse-chronology draws far fewer complaints.

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The complexity is an inevitable side effect of Facebook’s success. Facebook has not only become the largest social network in the world by a wide margin, it’s done so while aspiring to deliver all the things we want from the web in one simple news feed. But no company can offer an all-in-one service on the web and keep things simple. It’s simply the nature of the web, which has always tended toward the chaotic. It cannot be tamed.

The third vulnerability Facebook is facing is one that has nagged it since it went public. Facebook has two mandates — to create a service that will engage its users, and to make money that will satisfy investors. While Facebook’s presentation today played down the fact, the newly designed news feed is perfect for hosting video ads, a point that Fortune‘s Kurt Wagner noted.

The wider display of the news feed will further sideline the adjacent display ads that most people have learned to tune out. But they will open the door to the kinds of “suggested posts” and other ads that have been appearing inside the news feeds on mobile devices.

How intrusive these ads strike users will depends on the algorithms Facebook designs to insert them in feeds. They may be trailers or news content that appear because friends show them, or as Facebook faces investor pressure to beef up revenue, they could be simple ads. But one conflict that social media has yet to solve is the tug of war between what users want to see and what advertisers want to show.

So while Facebook’s new news feed makes some cosmetic fixes that users are likely to welcome in time, they don’t go very far in addressing deeper problems: rising competition from newer social networks, a bloated, complex interface, and the uneasy balancing act between users and advertisers. Facebook will be the leading social network for a long time, but it’s already starting to get harder to hang onto the crown.