Personally, I think FFXI is the best MMO out there but it's also the hardest and probably one of the most time consuming.
The learning curve is also pretty steep but luckily the general population is pretty awesome and will understand you're new to the game and most people will even gladly help you if you ask. FFXI is very heavy on the community so you don't get the little 14 year old twats you do in WoW.
You'll spend about the first 10 or so levels by yourself and after that you'll spend levels 11-20 grouping in hell (Valkurm Dunes) but once you make it to 25 the game gets pretty awesome. As long as you don't mind being forced to group with people, you'll love it.
Also, yeah, it can be quite difficult to find a group sometimes for certain classes but just stick with it and you'll find one. If you're one of those people that just sit there waiting for a group and do nothing you'll have a terrible time but as long as you pick up a craft or make you're own groups the waiting isn't bad at all.
As for the population level, I have no idea. Last I checked about six months ago it was still pretty strong (though not quite the strength of WoW) so you will definitely meet people. Just remember that while it's OK to mess up in lower levels and be an idiot, after about level 25 if you constantly mess up, do stupid stuff, rude, etc, word will get out about you and you'll quickly be hated by the server. Of course you're consistently awesome or even just good positive word will get out about you all over the server too.
