They would not have the space to keep it forever. There are over 1.3 billion people on Facebook and that number continues to grow.
Don't be worried about Facebook keeping your statuses (they don't give a damn). Be more worried (if you are going to worry at all) about friends taking screenshots.
Also, your statuses are fair game for advertisers, and they only need to be up for a second for it to hit the server. Ever wonder how and why you get pretty well-tailored (there are exceptions; they don't get it right every time) ads delivered to you? It comes from what you like, hide, share, comment on, join, and post about. E. g. if you post all the time about arthritis and your knee hurting you, and you like up a bunch of orthopedists in your area and like drug companies that offer painkillers, at some point in time, if the ads are well-targeted (some of the burden is on the advertisers to categorize their ads properly, and tag them well), then eventually you'll start to see ads for doctors who can do knee replacements.