Basically you'll be able to control your own data, sell it, and buy data from others. As you know, entities like facebook make billions literally selling your adspace to companies. They sell your data so that you're getting highly targetted ads, like if you 'like' a picture of a soccer player, you might see ads for adidas, stuff like that. Datum cuts out facebook as the middleman.
I believe a demo version will be out by Q1 of next year
That's the only issue I have with the project in the current form. I don't believe it cuts out facebook or others as the middleman, because they will keep selling your data. At this point, Medium seems like an alternative that needs to make itself interesting to buying parties in order for them to come to Datum and purchase their data there.
Think about it: Social media sites and email providers have very up-to-date info about you, which they can sell immediately. That will be highly targetable (in matter of seconds actually), which comes with a price tag. At Datum the user will have to keep their info/interest up-to-date themselves. I don't know how/if people will be doing that very often.
I actually really love the concept, but I do believe it needs some work (or a demo to make it a bit clearer).