Many sites I've been to implement some sort of shoutout system or wall on one's profile. This is a public discussion that you could have with friends rather than being restricted to one on one private messages. Every time you get one, you may get an alert, or not, like the achievements system already implemented. I would imagine it giving alerts, because some people may not go to the site for other than downloading some beatmaps or adding friends. Although not an urgent or large issue, when I joined osu!, I thought it would have something like this.