The one main thing about licensing your photo exclusively to one service or company is that it removes any opportunity to get a better deal now or in the future, through any other service or even by directly selling copies yourself. You're locking out any future potential revenue streams for that image. You may still be a photographer in 20 years from now but 500px may have disappeared or become irrelevant in 10 years.

That said, the logical reason for licensing something exclusively would be if you estimate that you will make more money out of one sale in the medium term selling the image exclusively than you would make in total, among any other future service selling non-exclusively, for ever. Obviously one single sale will net you a lot more money if you sell exclusively even than a number of non-exclusive licenses, but is it enough to beat the long-term earnings of small non-exclusively licenses over the lifetime of the image?

I can't really answer that for you for 500px because I don't know, and the potential audience for your images will also depend highly on the subject matter.