Ark is yet another stab at an old unsolved need: A site that searches all the social networks to find the people you're looking for. This is most notably not something that Google can do, as Google cannot search within the structured and closed databases of Facebook, Twitter, LinkedIn, and so on. (It should have better luck with Google+, assuming you're logged in.)
Previously I have covered companies like Spock (now iSearch), which aimed to do a similar thing. But many high-flying people search companies got the timing and the mix wrong: They peaked before Facebook was big, and they focused on helping users find public figures, not friends.
Ark has the stalk-your-friends thing going on, but it's not all bad. Looking for single high-school classmates, you creep? Or maybe people you're connected to who might be able to help you land that job at Amazon? The site lets you set filters like school, relationship status, employer, and so forth. Even if all you're searching for is someone in your Facebook network, it can do a better job of that than Facebook itself, which notably does not have a "search for all the single girls I used to know" feature. (See Zoosk, but that's another story.)
There will even be mobile alerts telling you when someone who matches a filter you've set is nearby.
Founders Patrick Riley and Yiming Liu told TechCrunch that they "imagined what would Google and Facebook build together if they weren't at war." And that's why neither Facebook nor Google -- nor Twitter or LinkedIn or Microsoft -- should own this company. The social networks will be at war for the foreseeable future, fighting for the richest databases of users. For one social-focused company to try to sit on top of all of them just won't work.
Can any big company do this, then? Palm sort of tried to integrate users' networks before HP bought the company and killed it. I suppose Apple could perform a similar maneuver. But I'm not sure if Ark works as a long-term independent play.
Because the open question is for how long Facebook and Twitter and Google et al will allow a business like Ark to crawl their data sets. The idea of Ark is to commoditize the social networks, which appears to run counter to the way they are working now. Furthermore, while Ark looks deeply useful, allowing people to do Girls Around Me-like stalking will set off flags.
On balance, though, it's a good feature for users and, to my mind, a good thing for the social networks themselves. It could well drive a user who's habitually on Facebook into a LinkedIn record when appropriate, or vice versa. It could help make the social networks more themselves. There's nothing bad about focus, and it would expand the social networks for everyone's benefit. I doubt the people running the networks will let it, but I hope they do.
See also the useful personal, cross-site search engine Greplin, which I've been using and recommend.