Facebook's official application for Windows 10 Mobile is a disaster - based on the Osmeta kit to convert across the iOS codebase, the end result is a whopping few hundreds of Megabytes and, unsurprisingly, a slow and bloated experience for the end user. At the other end of the scale is the Facebook mobile web site, which is fast and friendly. But does require all the browser trimmings to be in place, plus you can't do anything about actual content. So what's needed is a small and efficient UWP application, taking the Facebook web content and repackaging it within application form, stripping out ads and also rendering it within a dark theme, to save power on AMOLED screens. And, with a few small bugs and caveats, this is exactly what SlimSocial for Facebook is.

From the Store description:

Why download SlimSocial for Facebook?

• It is lightweight, less than 1 Mb;
• It has simple, modern design: there is only what you need;
• It is Open Source: the online audience this app code is on GitHub, anyone can verify the authenticity of the app and contribute to its development.
• It is free and without advertising;
• It is not intrusive;
• Respect your privacy: this app does not require special permissions. Facebook without consensus can not access any data of your mobile phone.

In terms of functionality, here's a brief walkthrough what SlimSocial offers:

Notice a few things: the lack of Windows 10 vortual controls - these get auto-hidden, to give you more room for content; the dark theme applied here; the optional space between posts; the lack of ads - and so on; (right) all of this re-rendering behaviour can be controlled in the Settings pane.

SlimSocial at work, making Facebook much faster and more power-efficient on W10M.

The utility's not perfect - when refreshing a page there's often a second or so of the unformatted original 'scraped' page and then the filtered, re-render kicks in. But it's still a heck of a lot faster than the official Facebook UWP.

It's also worth noting that as this is largely a web-based serving of Facebook, there's no native support for notifications, contact or calendar syncing. However, depending on your views on Facebook, these may even be a boon - do you really need pings every time someone posts on Facebook? 

You can grab SlimSocial for Facebook here in the Store. It's free, though there's an option to donate to the developer within the app itself.

PS. It works with Continuum too, though the rendering is very definitely optimised for phones. No doubt there's more to come here in future updates: