Arguably Tumblr is not a "Messenger" site, like the included socmeds are, so I don't know how willing they'd be to add it to the list. If that really bugs you, you can enable "custom site panels" at opera://flags/#sidebar-site-panel to achieve essentially the same thing. After enabling, restart Opera, then open the sidebar settings, scroll down to [Custom Site Panels], and add the URL of whatever socmed you want. The only differences I can pinpoint would be that the position in the sidebar is different, and you can't enable Opera notifications for custom panels.
