Apache Cassandra jumps to mind. It's the current elect NoSQL solution where massive scaling is concerned. It sees production usage at several large companies with massive scaling requirements. Having worked a little with it, I can say that it requires a little bit of time to rethink your data model to fit how it arranges its storage engine. The famously citied article "WTF is a supercolumn" gives a sound introduction to this. Caveat: Cassandra really only makes sense when you plan on storing huge datasets and distribution with no single point of failure is a mission critical requirement. With the way you've explained your data, it sounds like a fit.
Also, have you looked into redis at all, at least for saving key references? Your memory requirements far outstrip what a single instance would be able to handle but Redis can also be configured to shard. It isn't its primary use case but it sees production use at both Craigslist and Groupon
Also, have you done everything possible to optimize mongo, especially investigating how you could improve indexing? Mongo does save out to disk, but should be relatively performant when optimized to keep the hottest portion of the set in memory if able.
Is it possible to cache this data if its not too transient?
I would totally caution you against rolling your own with this. Just a fair warning. That's not a knock at you or anyone else, its just that I've personally had to maintain custom "data indexes" written by in house developers who got in way over their heads before. At my job we have a massive on disk key-value store that is a major performance bottleneck in our system that was written by a developer who has since separated from the company. It's frustrating to be stuck such a solution among the exciting NoSQL opportunities of today. Projects like the ones I cited above take advantage of the whole strength of the open source community to proof and optimize their use. That isn't something you will be able to attain working on your own solution unless you make a massive investment of time, effort and promotion. At the very least I'd encourage you to look at all your nosql options and maybe find a project you can contribute to rather than rolling your own. Writing a database server itself is definitely a nontrivial task that needs a huge team, especially with the requirements you've given (but should you end up doing so, I wish you luck! =) )