How many players are you considering to be "massive"?

Cryengine 3 doesn't include support for MMO scale networking, you can however use persistent data without any problems(not supported out of the box but adding in the ability for (trusted)third party servers to retrieve and send data to a central server isn't that difficult (I did that for the original half-life engine several years ago(For a bunch of CS servers)),
The (trusted(all controlled by the same CS community)) servers just fetched player data from the central server when the players connected (based on the players steam id) and sent events back when various in game events occured, This method is definitly solid enough to add things like global (or regional / community based) rankings, persistent character upgrades, etc to an otherwise standard 16-64 player FPS and doesn't actually require any support from the engine, (All you need is the ability for the game servers to open sockets), It does however require trusted servers, (if you let randomjoe host his own server you can't let those servers modify the central data, (you could let it retrieve the central data so that players can use their upgrades etc though)

For a MMO however (which really means thousands of players per game instance) you need to write your own backend if you intend to use Cryengine (the server thats included with the engine can't handle it)

The CryEngine is fine for developing content and immersing the player in a world.
I probably wouldn't push it to 100 players, though. This is based entirely on just reading about it, not trying it.
But if you have source access, you could probably remove the existing networking, and put in something massive-like instead.