This is a double idea.
I believe it would be nice if we could report, on the "Who's online" module, how many new members have joined since last login or since the user signed up first.
Even better and powerful for social pages would be if we could reference these as variables from mass-mail and Letterman for example.
The you blast your newsletter with a "teaser" like "Come see us again, there are XXX new members in our community since you last logged in!".
I believe this can make a site much more "sticky"
