Not sure if you'd consider StatusPage.io as a "clear leader". They claim to have around 2000 customers ( http://blog.statuspage.io/introducing-private-status-pages ) which I consider a small number of a potential target group. From some other posts, their revenue is around $3M/annually.

So if this is a $5M potential market, probably there is a clear winner. If this is a $100M potential market, things are just starting.