@Kardamom Obtaining copyright and protecting it are two different things, which is why the US Copyright Office offers registration despite the fact that creating the work is enough to confer copyright. Posting the photos on Facebook would be evidence in favor of your claim, but they would not constitute proof in the event that the issue came before a judge (or, worse yet, a jury). If your evidence is better than the other side’s evidence, though, you generally win. So if they don’t have evidence that they created or posted the photo before you did, they’re out of luck.
The thing to ask yourself, though, is what you want to protect. If all you want is credit for your work, then watermarking is probably the best way to go about it. The watermark identifies you as the creator, and will likely be enough to get most websites to comply with a takedown request should you wish to submit one. If you want the pictures distributed widely, you could also submit your work to stock photography websites that require anyone using the photos to credit the source and creator (like in newspapers when a photographer is credited in the caption). My wife does this, so I could probably PM you some information about it.
If you’re not willing to go to court over your photos, then registration is a waste of time and money. It won’t get you anything that watermarking and takedown requests can’t get you for free. Since you mention photo contests, though, keep in mind that some contests make you surrender some or all of your rights in order to submit your work. Always read the terms and conditions carefully. Or just Ctrl-F “copyright” to find the interesting parts!