That's like saying Apple is worth $600 billion, Motorola is worth "only $12 billion", but has 1/10 of Apple's market share, so therefore it must be a
great deal to buy Motorola. Wait, actually that's exactly what Google thought - and they quickly saw the error in that sort of thinking after they purchased it.
Do you see how that argument is flawed? Leaders in a market tend to be worth much more and have much higher profits than even the 2nd player in the market, let alone the third or fifth. When you have a small percentage of the market, it's also much more difficult to make something out of it, than it would say for a market leader to go from 30% market share to 50% market share (and billions of dollars more in revenue).
We could repeat the argument for a small social network that has a much lower market value than Facebook per user, and so on.