Which NoSQL database(s) should be used for advertising platform?
To be very fast, the database should only use memory and not disc since disc operations usually takes much longer time. But then your data aren't persistent in case of a crash. What you could do is assynchronous disk operations, by that most of your writes will be persistent but it's not guaranteed that the last few writes are persistent. If this is okey for you, you can use Redis. See also Redis - The Hacker's Database.

If you really care for persistent data, you should use a database that have been tested and used for years. Then PostgreSQL is a good choice.

To achive high availability, you need a distributed system with multiple nodes, so they can tolerate that at least one node goes down. Then I would use Riak.

The question is why do you need NoSQL? That word means almost nothing. I would recommend you to go with a traditional RDBMS like PostgreSQL until that can't handle your problem. It will be much easier to handle for you and it's much more flexible.

What you can do to improve the speed is to use caching. Then you can use Varnish Cash infront of your server, that will improve your speed a lot.

See also What are the differences between NoSQL and a traditional RDBMS?