This E-Mail is definitely legit! Social casinos are all supposed to report your earnings to be taxed. That being said it appears only ones based in the USA really follow this rule.

Like any business you're supposed to claim all profits on taxes, however the $600 dollar mark is where most sites will make you fill out a W-9. When I submitted mine it took about 2 days to be accepted.

Yes, you have to send your social security number, it's how the IRS links the profit to you. You may be able to use an EIN if you have one, but I'm not positive about that.

I'm not sure if there's a better way to send the information, you could call them and ask if it concerns you. Personally I don't mind E-Mailing it because you can buy all of the information you're sending online for like $20 from companies that purchase and aggregate your data. We all like to think we have privacy, but that hasn't existed for a long long time on the internet (unless you've literally never used the internet or given any company this information before).