How active is this game online?

I just ordered this game for PS3 (I believe it's only for PS3, but I never bothered to check), I know zero things about this game apart from a bit about a mad monk boss that can be a human player or something. I loved the DLC for Dark Souls because for that bit, I was going entirely blind. Not sure why it took me this long to get into Demon's Souls. So, how active is the online community for Demon's Souls? Enough to get possible summon help, get invaded, invade, etc, without too much effort? Or should I expect a single player experience unless I request a bit of help?

Well, I look forward to it this Wednesday then. I have no free time until at BEST Wednesday, so I don't mind the delivery time of it in the slightest. I have a sudden urge to look up every single boss and weapon in the game, but have to resist, lest I ruin the surprises bound to happen. Don't even know what the tutorial boss is. Only ever saw the opening movie for Demon's Souls. 

Just wondering now on how the gameplay compares to Dark Souls, if it's similar enough to jump right into with little to no difficulty, or if I'll be struggling for a bit. Also wondering if I'll do a strength build first, dexterity, or a mixture... depends on my mood when I get it :p

Another thing you could do if it seems a little boring because you're not finding summons is post in the help section and try to get a co-op buddy that can help you go thru and possibly show you where everything is so you don't miss anything.

I wouldn't do that, I only summon for bosses that give me too much trouble too many times, or after I already beat the game, and want to co-op full areas for the hell of it. Certainly look forward to the game this Wednesday, I'll be looking forward to it during my long work hours. If I work, rain might not allow us to work... meaning I might have several days of no Demon's Souls, but no work. Torture :p

You can find some co-op partners on lower SLs, but this depends on your region. On EU with SLs 1-50 I always could find at least one summon sign, but it took a few minutes sometimes. Also, on this SL, if you put your sign down you'll get summoned pretty fast and can ask the person to assist you in return.

You can always try asking for help in here, but other than that, prepare yourself to play single player for most of the game.

i was just playing yesterday and came across a few players. Only one invader. But still. Considering how old the game is, its still arguably active. I STILL get more summons in demons than dark due to a better server/connection design.

most player don't set their sign before the boss fog, but more at the start of a level.
as said, don't over level, around 30 for the x-1 worlds and around 50-60 for the x-2 worlds and 1-3 and 1-4
haven't played NA version for a while, but when i left it was only a bit lesser multiplayer as in the EU version.
in pvp be prepared for two sorts of foes:
1. DeS expert - will kill you pretty quick (1/4 to 1/5 of the player)
2. DkS expert who thinks is unbeatable and dies really quick (taunt 'em)

I played a bit over the weekend on my sl36, was summoned at least 20x in 2-1 and 4-1, and we were also invaded 5-6 times (sadly, mostly by regen trolls with scraping spears).  It was actually more active than I remember, lots of bloodstains and ghosts too.