It's not the low population. Is the P2P system them have for hosting games, instead of the dedicated servers that Demon's Souls had. Coupled with other factors like the, honestly, ridiculously narrow level range and the fact that nobody really has ANY incentive to play as a Human instead of a hollow...it turns the whole experience into a ghost town even though there are likely millions of players out there.
Playing with a friend or anybody specific is down to luck, hoping you get put in the same random choice of clusters when you turn on the game.
Unfortunately THAT is the one thing they can't fix with a patch. You don't just patch in dedicated servers.

Lots of good points here. Lots of people are playing, but were just not connecting w/ each other.
For example, throw on that cat's ring. Sometimes your getting summoned once every 2 min. Other time you go 20-30 min w/out being summoned.
In human form, you head to a boss door. Sometimes you'll have a ton of signs, other times very few.
Most of the time I don't have too much trouble summoning in the predictable places, but invading or being invaded is few and far between. I think I've been invaded less than 10 times. As far as invading w/ a cracked red eye stone MAYBE 3 times (haven't joined the wraiths so I haven't been able to attempt it very much w/ the limited supply)
Honestly FROM has confused the hell out of me. They put in all these great covenants that are obviously geared around multiplayer. But the p2p system has put a crutch on the entire multiplayer system. I also thought they'd improve the lag issues as well, but the p2p system seems to have made it worse since your not always connecting locally. Yes I want to play internationally, but not at the cost of getting lag stabbed by everyone not in my direct locale
Some say that the multiplayer works as it was intended. Random meetings in a vast and lonely world. Though I can appreciate that vision, and I do feel that the vision is fitting for a game like dark souls, the bottom line is that people want to game with their friends.
I really hope they can do something to fix this. This game is such a vast improvement on demon's in so many ways, yet these tech issues seem to be holding the game back. Keep those fingers crossed.