A Syndic8 fan asked me to analyze the number of distinct domains represented in the Syndic8 feed database. A few SQL queries later, and I had the answer: 15,833 unique top-level domains. Any domain which supplied more than one URL (e.g. blogspot.com, blogger.com, and so forth) was counted just once.
Here are some of the top domains: <table style="border: 1px solid #eee">
At some point I will add these to the official collection of Syndic8 Feed Statistics, and I will also allow for feed retrieval by domain name.