I don’t think lemmy as a whole can be sold out to Facebook since it’s open source, right?
Yes, but lemmy is even more resilient. It is licensed as AGPL-3.0.
That is a copyleft license which requires anyone using the code publicly (e.g. to run a proprietary server) to make their code public and permit free usage of it. And they must not change the license.
I don't see the point of buying something which is already freely available to everybody, especially since you are not allowed to hide it, even partially, or modify the free nature of it.
What could more reasonably be sold is data, visibility and userbase (big, active instances).