Tiana's is on the sign - I think it's easy to miss, especially if you're coming around from Pooh.
For me, Tiana's single rider is CM-dependent, too. Some of the CMs will do what they can to match up standby parties into groups of six - the SR line might be short, but it could be a half-hour or more if they're able to pull that off. Still probably better than the standby line on an average day.
Anecdotally, Space Mountain's line has seemingly gotten a lot longer since they moved it out of the hallway and to the outside - it's a lot more visible than it was. It was almost a hidden secret before, but I didn't even bother to get into it on my last visits in January (on what were relatively quiet days). It can also be CM-dependent - occasionally, they'll just forget it's there. (It's sometimes frustrating to stand there and watch a train launch with empty seats.) Again, probably still faster than the standby line.
Frankly, I'm not sure I'd ever want to use Soarin's SR. The few times I have (or just been in line as a single), I've ended up in the worst seat in the section. (Not always, but it feels very hit and miss.) I'd rather hit it in the morning with a short standby line - that way, I've got the option to ask for a more desirable section/row. (I'm a little bit picky, though - I'd rather wait for the next showing than be way out on the edges.)
I'm a big fan of the other SRs, though.