High frequency trading is Solarflare's original market (and Mellanox has their InfiniBand market), but both of them are becoming more and more common in the commodity server market as well. Particularly since Intel dropped the ball on 40G (and REALLY dropped the ball on 25/100G), and Broadcom is out of the adapter market, there is a void that other vendors are filling.
When you're spending $30,000 on a server, it doesn't really matter if you spend $1200 on a network card. Those CPU cycles and storage bytes have to go somewhere to make money.
Not yet. They launched 40G a while ago, but some issues have kept them from the same kind of dominance they had with 10G.
25G is supposedly coming soon-ish, but 100G is still 1-2 years away. It's going to be hard to compete with vendors who shipped their first products in 2015/2016.
Meanwhile they have a 100G OmniPath adapter, but who cares?
When you're spending $30,000 on a server, it doesn't really matter if you spend $1200 on a network card. Those CPU cycles and storage bytes have to go somewhere to make money.