SQL Server vs. NoSQL
I don't think so you have to move your database from SQL to NoSQL unless and untill you are serving thousands of TB data. If you properly normalize your tables and serve the data and also need to set proper archive mechanism it should work.
If you still have question what to choose and how, than check this. Let's assume that you have decided to move on to NoSQL database than there are lot of market player. Just have a look at the list which is again depending upon your need and type of data you have.
With a simple table structure and data that fits on one server, it doesn't matter much what platform you use. There are a several possible reasons to need to move to NoSQL:
Data scaling - SQL works best when all the data fits on one server (up to a few TB). The reason a lot of NoSQL stores don't have join is that they were designed not to require all the objects to be on one server.
Performance scaling - NoSQL stores do tend to be faster at handling high traffic, but not necessarily by enough to matter. You can improve SQL performance quite a lot with replication and caching as long as you aren't running into data size issues. Writes generally do have to run on the one server, but in most cases you will need to improve read performance long before write performance becomes an issue.
Complex data access - some types of queries simply don't fit well into a relational model. Graph and set stores work quite differently from relational databases so are a better fit for some applications.
Easier development - If you don't already have a SQL database and all the code to support it, using a schemaless datastore can save quite a bit of development time.
Sql Server scales pretty well. For example, Stack Overflow used it to serve you this very page. Facebook and Google might use a form of nosql, but even if you make it really big you're unlikely to rise to that level.
Actually Facebook use a relational database at its core, see SOCC Keynote Address: Building Facebook: Performance at Massive Scale. And so do many other web-scale sites, see Why does Quora use MySQL as the data store instead of NoSQLs such as Cassandra, MongoDB, CouchDB etc?. There is also a discussion of how to scale SQL Server to web-scale size, see How do large-scale sites and applications remain SQL-based? which is based on MySpace's architecture (more details at Scale out SQL Server by using Reliable Messaging). I'm not saying that NoSQL doesn't have its use cases, I just want to point out that there are many shades of gray between white and black.
If you're afraid that your current solution will not scale then perhaps you should look at what are the factors that prevent scalability with your current solution. Test data is cheap to produce, load the 'exponentially increased' data volume and run your test harness, see where it cracks. None of the NoSQL solutions will bring magic off-the-shelf scalability, they all require you to understand how to use them effectively and deploy them correctly. And they also require you to test with large volumes if you want to ensure success at scale. Same for traditional relational solutions.