Watching a YouTube video while surfing the web is real multitasking, and easily achievable on the LG G3. Like the "Multi Window" feature on Samsung Galaxy devices, the G3 has the ability to run two separate apps simultaneously with its Dual Window feature.
Unfortunately, you're only limited to stock, Google apps in Dual Window mode, so playing a game and watching Netflix isn't a possibility.
Thankfully, developer Abdullah Gheith wasn't having it and developed a mod so all G3 users could add Dual Window compatibility to third-party apps. In order to install the mod, you will have to have a rooted LG G3.
The mod can be found on the Play Store for $0.99 using this link, or you can check out the developer's thread on XDA to get a generously offered free version for testing. The paid version does offer the ability to hide LG apps from your list, so if you want the full-powered version, I'd go with that one.
Open the app and grant it Superuser permission, then select the apps you want to use in Dual Window, hit Apply, then Reboot to save.
Open the Dual Window option and your selected apps will be available to use—just drag and drop them like you normally would, and use Twitter while you Instagram, or whatever else you want to do.
If you love doing two things at once, then you'll want to give this a try. Let us know what you think, and which apps you like using in Dual Windows, in the comments below.