We just added a new option for those of you wishing to rearrange your
sharing habits in light of the new features. Now, the "clear all
shared items" link also includes an option to apply a tag to all of
these items. That means you can safely clear out your shared items,
but still keep them organized together, either for your own reference
or to share with a more limited audience.

You can find this option by going to the "Friends" tab under
"Settings," then clicking the "move or clear your shared items" link.
The tags menu, should you choose that option, will let you select an
existing tag or create a new one.

Regarding friends you haven't chatted with: As Zac T noted, what
you're seeing here is probably a side effect of a Gmail feature. Go to
your Gmail settings and click the "Chat" link. You'll see an "auto-
add" option there that adds people to your chat contact list when
you've emailed with them. That bypasses the ordinary approval step you
have to go through to chat with someone (on the assumption that it's
okay because you're already communicating with them). You can turn
this option off if you're not comfortable with that.