Remember that for web, the resolution doesn't matter a single bit. It could be 300, or 72, or 1000000000, or 12. It's completely irrelevant to the file.
The three standard Facebook sizes are 720, 960 and 2048.
720 is very small for these modern times, and I don't think anyone uses it any more.
960 is also a bit on the small size, but is still adequate for most browsing, and of course is quite quick to load. Also, it's too small for anyone to steal and print (print nicely, anyway).
2048 is big enough for everyone's screen, no matter what size, but of course it takes the longest to load, and it's VERY printable, so if you choose 2048, make sure you watermark prominently.