"Those who have once had the faith, and fallen away, are subject to the loss of their salvation."
Two points.
1. I believe the Bible teaches the reality of apostasy.
2Timothy 2:12
If we suffer, we shall also reign with him: if we deny him, he also will deny us:
2. It is not clear from the passage in Heb. that the people had true faith. They may have been like the crowds that followed Jesus, partaking in Spirit to the extent that they witnessed his awesome miracles and yet, in the end, turned away. Interesting to note that the author is quick to reassure the the reader with...
"Though we speak in this way, yet in your case, beloved, we feel sure of better things--things that belong to salvation."
I see a strong allusion in v8 to the parable of the 4 soils.