Facebook polls are neither secret nor verifiable. Facebook knows how everyone voted, and voters cannot verify that Facebook is reporting the correct result. Those problems can theoretically be solved with cryptography (assuming the voter has full control of the device used to cast their vote), but not in a way the average voter can understand and verify.

Another problem with online voting is that malware can infect a voter's device to spy on how they vote and/or change their vote. I don't think there is a solution to that problem.

Online voting is a terrible idea. The only way to have secure elections is to publicly count every ballot by hand. If you think that's too expensive, you think electoral democracy is too expensive.