Dropbox.com, the privately owned, cloud-based repository of too-big-to-email files, had some bad news Friday for any investors who've been hoping it would soon get around to doing an IPO. Rather than going public itself, Dropbox is taking somebody else private.

Specifically, Dropbox confirmed yesterday that it's bought the privately held email management app company called Mailbox. Mailbox had been busy attracting funding from private equity to fund its rapid growth, but when approached by Dropbox with a deal this week, it quickly dropped its plans to go it alone.

Why? It's hard to be sure. Perhaps Mailbox was simply impressed by the "profoundly talented bunch" of programmers at Dropbox, whom they complemented on their website. Or maybe their motivation was of a more pecuniary nature, and Mailbox was won over by the $100 million in cash and stock that Dropbox reportedly offered.

Whatever the motivation, though, Mailbox confirmed yesterday: "Rather than grow Mailbox on our own, we've decided to join forces with Dropbox and build it out together."