Are you actively participating, or just looking?
If you are just looking at content anonymously, such as looking up a published tutorial on Reddit, then an alternative front-end may prevent the website from tracking your behavior.
This, of course, depends on the endpoint being used. Even if a front-end like "Libreddit" is open-source, it needs to be hosted by some publicly accessible endpoint, in order to mix the behaviors of many different users together and thus hide every single individual. So there could be an endpoint like
libreddit.somehostingcompany.com, which thousands of users use. So even if Libreddit itself is open source, you can't ensure that the hosted endpoint is using exactly that source, and isn't collecting any information on their own. In fact, it's overwhelmingly likely that they log and monitor at least some communication, even if just for the purposes of keeping bots out.
If you are actively participating, the front-end matters way less than otherwise. If you are registered on a social network like Reddit, then they have information about you - when you first registered, when you last logged in, which content you published, which posts you interacted with and how.
Even if your front-end aims to keep as much of your viewing behavior hidden, your interactions are necessarily tied to your identity.
Would it be possible to circumvent this? It might be. You could register and authenticate with thousands of accounts, and whenever you performed an action, such as publishing a post, upvoting something, etc., that action would be performed by a randomly chosen account.
This would make it more difficult for the social network to track you as an individual, but it is very difficult and time-consuming to set up.
What information are you worried about?
A lot of information sold by social media companies is "aggregated media". For example, they may be aware that a thing called "Touhou Project" exists, and that the images you upload are related to Touhou Project. You may even voluntarily tag them as such. The social media company labels your account as "Someone, who is interested in Touhou Project."
Other people may interact with your account a lot, who are then also labelled that way. But the social media platform may detect that a significant amount of these people are also tagged as "Someone, who is interested in the Fate franchise".
This is marketable information: "Many people, who like this thing, also like that other thing." This can be used in advertising, in order to make advertising campaigns much more successful. After all, if I was a fan of both Fate and Touhou, and someone would advertise a figure of a popular character of either franchise, chances that I would buy it is magnitudes higher than as if the average user were advertised said figure.
The thing about this information is that it's nigh impossible to prevent a social network from getting some idea of what you are into. If you post about politics, they will know you care about politics. If you like a post about kittens, they will know you like kittens. Anything you interact with will be an indicator of the things you care about, be it positive or negative.
If you are worried about more personal information, that is luckily a little bit easier to control. A social media company has a lot of data to work with, but it isn't magic - they can't know where you live, unless you publish it somehow. Either directly, or through indirect means.
For instance, if you constantly publish images of Ljubljana, chances of you being Slovenian are pretty high - especially if some of the posts you made were in Slovene. If the images you uploaded contained location information in the EXIF data, then they'll probably be able to pinpoint your location much better.
Note that none of this is related to the front-end you use in any way. It's about the information provided by an account you identified as - regardless of the
front-end in question.
Alternative front-ends can prevent a social-media company from collecting some information about your behavior, but can enable the host of said front-end to collect just the same information.
Once you attempt to actively participate in the social network, all bets are off.
Note: I'm picking "Libreddit" and "Reddit" respectively as examples, but the same holds true for any other alternative front-end and social network.