The Mass fabricator is an economic unit. In Supreme Commander, Mass Fabricators employ the principles of mass-energy equivalence to create mass out of energy. The energy costs are, naturally, very great, so mass fabricators (often contracted to "Mass Fabs") are generally used when the limit on the mass obtainable by conventional means has been reached.
Gameplay changes in Forged Alliance have deterred the use of mass fabricators as an economic staple. Mass fabricators can be first constructed upon reaching tech level 2, instead of early construction by tech 1 engineers. The production and energy costs have been raised, while the mass output has been cut in half, making turtling more difficult and forcing the player into territory control battles for access to more efficient mass extractors.
The fact that Fabricators can be built almost anywhere allows you to use them to give factories bonuses. However their power requirements means that you need a large number of power plants. Make sure you have plenty of power before building them, and putting them next to tech 3 power plants makes it easier to maintain them. You need two tech 2 powerplants for one tech 3 fabricator.
Because of their sheer inefficiency, Fabricators should only be constructed when all other methods of improving mass production have been exhausted- all Mass Extractors should be upgraded to T3 and surrounded with Mass Storage and all large stores of reclaimable mass should be utilized first.
Think Small Edit
If you find the need for mass fabricators in Forged Alliance, you're much, much better off building several Tech 2 Mass Fabricators rather than 1 Tech 3 Mass Fabricator, considering the "cost-benefit" analysis. There are several reasons for this:
1. Keeping the mathematics to a minimum, 12 Tech 2 Mass Fabricators get you the same rate of mass from 1 Tech 3 Mass Fabricator, for only 1700 (-49%) less power. Whereas 24 Tech 2 Mass Fabricators get you TWICE the mass that you receive from 1 Tech 3 Mass Fabricator, for only 100 (+3%) more energy. If assisted with power generators, mass storage units, etc., you get similar results. As another comparison, one Tech 3 Power Generator is not nearly enough to cover the cost of a Tech 3 Mass Fabricator, whereas one Tech 3 Power Generator is enough to cover the cost of 15 Tech 2 Mass Fabricators, not including bonuses
2. With a Tech 3 Mass Fabricator, all +12 mass is in that fabricator, so if your opponent destroys it, that's it. But building several Tech 2 Mass Fabricators allows you to spread them out, so that each one destroyed only decreases your total mass accumulation rate by 1. Note that this advantage is nullified if you don't spread them out, since they will chain-react upon destruction.
3. Tech 3 Mass Fabricators are MUCH MORE volatile than Tech 2 Mass Fabricators. A Tech 3 Mass Fabricator, therefore, must either be built out in the open, or be more carefully guarded, as it can damage the majority of a base if destroyed.
As for disadvantages, Tech 3 Mass Fabricators:
1. Are quicker to build than several Tech 2 Fabricators
2. Take up ever so slightly less energy when surrounded by power generators as opposed to several Tech 2 Fabricators surrounded by generators
3. Don't destroy each other when several are built in a row.
4. Take up MUCH less map space if limited construction area is an issue. This also makes it much easier to fit them under shields.
5. Count as only 1 unit towards the maximum army limit of 500 units whereas 12 Tech 2 fabricators count as 12 units, resulting in 11 more slots being used up for economy.
6. Generate 1.3 Mass per Tile as opposed to one T2 Fabricator generating 1 Mass per Tile. (9 Tiles/12 Mass = 1.3 Mass per Tile.) This can be further increased to 2 Mass per Tile with the Maximum Adjacency bonus. (9 Tiles/18 Mass = 2 Mass per Tile)
7. Can recieve a Bonus from Generators and Storage Units whereas T2 Fabricators can only recieve a Bonus from Generators.
But the advantages for Tech 2 generators outweigh the disadvantages, so you will most likely find that building several Tech 2 Mass Fabricators is better in the long run than building one or two Tech 3 Mass Fabricators.
16 Tech 2 Mass Fabricators surrounding a Tech 3 Power Generator will net you +16 mass while using -1936 energy. Building a single Tech 3 Mass Fabricator next to a Tech 3 Power Generator will net you +12 mass while using -2843 energy (A Tech 3 Power Generator only generates +2500). Comparing these two, the 16 Tech 2 Mass Fabricators are cheaper to build by a total of 1400 mass and 1000 energy, produce +4 more mass, and use 907 less energy than the single T3 mass fabricator. So from an economical standpoint Tech 2 Mass Fabricators are completely superior.
A Tech 2 Mass Fabricators health is 360, while its death explosion deals a damage of 370. This means that if a single Tech 2 Mass Fabricator explodes, all surrounding Tech 2 Mass Fabricators will also explode. Tech 3 Mass Fabricators do not suffer from this problem.