Before anything, you should keep in mind that there are very strict guidelines for brand usage, including:
Don’t modify Facebook brand assets in any way, such as by changing the
design or color
(From Facebook Brand)
LinkedIn is quite similar:
Logos may be downloaded for use in reporting on LinkedIn's business.
These logos may not be altered, combined with other marks or used in a
misleading manner. Your use of these logos is subject to your
compliance with these Guidelines.
IN Twitter's case, you wouldn't have any problem with shape. And arguably, with the whole icon just as you have it, since you could say it's white. For more info, check Twitter's brand guidelines, specifically the section Misuses
So, there's a legal aspect about NOT fooling around with this
The only truth is reality. And reality shows that everybody and their mothers play around with social media icons at will. And as far as I know, these companies never cared very much. They could simply go to Dribble, Freepik, Pixeden, Font Awesome or whatever big graphics resource and tell them "stop what you're doing". Yet they don't. So... while the above point is worth noting, I'd say it's a bit of "as long as you don't go bananas, feel free to play a bit".
All the above being said, you should consider this: a brand is made of its components. Between those components, you'll find brand iconography in the form of imagotypes, logotypes, isotypes and/or isologos. All these are made of a basic component: SHAPE. Everything else such as color may or may not exist, but shape is the only thing that exists in every brand type, even if it's just a logo (where logo is just words, without signs). Thus, by altering the shapes, you alter the perception of the brand. And this is why all important brands have specific guidelines, as shown above.
In your specific case, where you don't alter shape, but alter color and context, the loss of brand recognition is certainly smaller, but still exists. Which, again, takes us to the brand guidelines above (surprise!).
If you take a look to those guidelines, you'll also see how the icons should be displayed in context, which includes spacing. So, if you see the circles and the rounded corners squares, you'll see spacing is not the same. Therefore, you'll be breaking a guideline. But getting to your questions, the square with rounded corners is probably your best choice. The circles encapsulate the icons in a tighter way, leaving very little space between the icons and the backgrounds
The process of changing the icons falls into the debranding realm (and an unsolicited debranding, as we're at it), so you'll need to ask to yourself: is that social media important for you or not? If the answer is YES, then follow the guidelines. After all, they were built by top professionals and have lots of time of testing. Otherwise, if you want to make your brand stronger by debranding the social media icons, you can play a bit