Facebook announced last night that it was adding search to the Inbox feature of its site. According to Facebook engineer Prashant Malik, Inbox search was something the company knew they needed for a long time, and was heavily requested by users. Search is one of the fundamental features that any email-like messaging system needs, and adding it is significant for Facebook because it takes the Inbox app once step closer to being a viable email replacement for users. Facebook is already the ultimate address book for many users, so why not make it a more usable messaging tool as well?

The reason it took Facebook so long to add search, said Malik, is that the company “wanted to take the time to make sure we built the right solution that would scale to support everyone using the site.” MySpace recently adopted Google Gears to encourage users to search and sort their emails offline, which will almost certainly make the service easier to scale.

Image via Inside Facebook.

Facebook’s Inbox is still a long way toward being a “Gmail-killer” — or any other full email option killer, for that matter. Though the site does email full copies of messages — a practice it began last December — and though it lets users send messages to email addresses, it still lacks many of the “must have” features of an email client. Message organizing features, for example, are almost non-existent, as is the ability to reply to all recipients of a group message. There’s also no way to reply to message sent from Facebook via an outside email client (i.e., the way you can reply to Basecamp message by replying to the notification email), nor is there a way to send messages to Facebook users from outside email clients (the latter is an effective type of spam control, however).

For some younger users, though, messages on sites like Facebook and MySpace have already supplanted email as the defacto method of asynchronous communication on the web. Our own Bernard Lunn argued last week that LinkedIn could do the same thing for him and Outlook by adding more robust messaging features. I’ve long argued that Facebook could transition into a network suitable for business, and better messaging capabilities is definitely a step in the right direction.