Have two level 50s, a level 39 and a level 40 (if it wasn't for Hoth, I'd most likely have 4 level 50s)...and I don't do space missions. I did a few with my main, but to me, they are boring as hell.
Fastest way I've found to level up is to do missions, do bonuses and give your companions lots of gifts. At level 50 now and was 2 to 3 levels over on every planet. Hell even at 50 she is over leveled compared to other fresh 50s (mainly thanks to mav and my main feeding her equipment and mods).
I’m sure you could probably level just through space missions and flashpoints, but it would bore me to death. Even the worst story line in the game (IMO the Jedi Consular) is more entertaining than space missions.
I know mav does, for the most part, just the class story and then does dailies to reach 50 as fast as possible. Not my cup of tea, but it works as he has the most 50s in our guild.