So from what I've been hearing about ESO is that it's just one huge mega server and have one really important question. Will PS4, Xbox One, and PC users be on the same mega server since there is technically one server?

They have separate servers for PC/Mac, Xbox One and PS4, so technically it's three mega servers.

Oh, okay. I kind of wanted to play with people who have Xbox One and PC because I'm skeptical about how many people will be playing ESO on PS4.

I understand what you mean, and sure, the ps4 and Xbox one servers will probably be less populated than the PC server, but the mega server phasing will most likely always put you with other people, so I don't think there'll be a shortage of players.
And when you think about it, it does seem necessary to keep the platforms separate, as the console isn't really designed for MMOs, whilst the PC has the keyboard and hotbars, the console you have the controller, of course then you could attach a keyboard and mouse to the console, but that is something they may have to tweak for the console version, as it would seem PC players have an advantage, so keeping them separate is a good idea.
As for Xbox One and ps4 being on separate servers, well that's just Microsoft and Sony being unable to cooperate with each other.
But consoles players have a benefit with the game being released in June rather than the PCs April release, ZOS will be able to polish out any bugs or glitches for the console release, hopefully.

I know what you mean

Well you should not fear the small communities by them being small. I'm not sure how nerdy you are regarding your MMOs or any one else reading this topic for that matter but there are usually small pirate servers emulating old or older versions of an MMO. You can find them for SWG and EQ and what not. Usually this small communities are allot more fun to play in because they are small. So what is that good about small communities you might wonder. Well in small communities every one nows every one, more or less. This making them allot more friendly or maybe comradely is maybe a better word for it.

I really miss the days when MUDs was popular in many ways because of this. Most of the MUDs back in the 90's had only around 500-1000 players but I think this is what made them so popular as it forged grate friendships and rivalry something official MMORPG servers never managed to do in the same way.

I'll be honest, I haven't actually played an MMORPG so ESO would be my first! Well, when it is released that is.