The entire market is currently being propped up by unseen forces. There doesn't seem to be a stable sector holding the load, like housing or manufacturing, so it's hard to tell why they are still rising. When this has happened in the past, it was due to some "bubble" in one sector, which inevitably bursts, and causes a drastic downward trend. (recession/crash/etc...)
I would be reluctant to be in this current market with any meaningful amount of wealth.
As for FB in particular, their ship may have sailed as far as stocks go. They were grossly overvalued when they went public, and with recent acquisitions, are only now reaching the point where their value is justified.
You also have to ask yourself about their long-term viability. For ANY tech stock, this is risky. Anyone who owned IBM, Dell, Apple, or Microsoft 25-30 years ago would have one perspective, but anyone who rode the 90's tech bubble roller coaster would have another. You need to look at the base customer that this company serves. What drives their business? Are they fickle? Could they abandon the service with little notice? Would you have enough time to get out before they did?
With an aging community joining, the user base is likely becoming more stable. However, the base as a whole is still largely teens and young adults. Will they stick around, or move on to the next big thing in a year?
It's hard to guess.
I didn't think it would survive this long, yet it's still a thing.
It's your money. Do what you wish. You're the only one that will answer for it. Take time to study their trends, and the company's future plans. If it makes sense to you, go for it.
As a prepper, I just can't give an outright "yes" to any market investing when I see an economy that can't sustain growth, yet "growth" is what we're being shown.