Yes! Unity is certified as safe by McAfee Site Advisor.
If you are worried about downloading the plugin from Facebook, try downloading it from the Official Unity Website.
Unity (also called Unity3D) is a cross-platform game engine developed by Unity Technologies. It is used to develop video games for web plugins, desktop platforms, consoles and mobile devices, and is utilized by over one million developers. Unity is primarily used to create mobile and web games.
If you would like more information on the safety and security of Unity Web Player, check out their Security Sandbox page.
Update October 2013: Recently Norton Anti-Viruses (and those that use information from Norton) have mistakenly listed the Unity Web Player as unsafe. This is incorrect, and should be resolved on Norton's side shortly. Until then, if you notice issues when attempting to load Slots Central, try disabling your Norton or other anti-virus program before you play the game - but remember to switch it back on when you're done!