I think the social aspect isn't necessary for addictiveness. I can coordinate an event or meeting via WhatsApp or SMS or even snail mail (weddings), but there's nothing intrinsically addictive about that. To put it in your words, these are "transactional" activities towards an offline goal.
The way I think of it is that there are two mechanisms that make Facebook addictive:
The first is voyeurism. We can see similar impulsive behavior when we think things like tabloids and gossip, as well as the psychology of clickbait (e.g. "you won't believe what this local mom did"). People are just naturally curious to know what's going on with other people.
The second mechanism is validation. We can see similar dynamics with how youtubers track "success" and "progress". Twitter presences basically operate on a similar principle. Instagram is another example where people feel compelled to share somewhat artificialized "slice-of-life" photography to audiences that may or may not be their circle of friends. There's karma in HN/reddit/stack overflow, etc and stars in github. In Facebook, it's "likes". The common factor is that people like seeing a little number go up in response to some activity and are compelled to re-engage in said activity due to the positive reinforcement.
So I'd argue that an alternative to Facebook already exists - in the form of Twitter, Reddit, Instagram, clickbait farms, messaging apps, etc. The only stronghold I see left for Facebook is that it's the default place to broadcast random things that don't fit into any other platform (i.e. what's going on with who), and even that is eroding with the whole aunt-and-college-buddies-in-the-same-network-awkwardness thing. The only other thing going for Facebook is that it's pretty much the only platform that enables chain mail sort of stuff (you know, that uncle that is always posting political articles or alternative medicine things, or inspirational quotes or whatever) since it sorta automatically gives people an audience to broadcast to (for the validation aspect).