What are the negatives of social media sign in?

Question

Social media sign in has become popular to use on websites, but what negatives are there with its use?

Does integration add trackers to your site?

Does it slow overall performance?

Are some social media logins better than others?

I haven't found much info on this online, and all the data I've seen is on conversions and marketing. I'd love to hear facts from the development side.

Edit: I'm feeling confused by the downvotes. How is my question bad or irrelevant? Social sharing buttons were all the rage but most people agree now that it isn't worth it, even though it seemed at first to have great results; and from the development side, it slowed page loading and added trackers to our sites.

The companies most excited about the buttons before, as I recall, were companies selling ways to add a ton of those buttons to your site; and most of the advocates I see now are similarly marketing products that allow you to add a ton of buttons. I'm asking what login does from an angle other than popular marketing.

Answer 1

Using OpenID for login is great because you don't have to remember many different logins, and even though there are some minor problems, I don't think you shouldn't use it (I'll use Facebook as an example here):

1. The user has to trust you. Maybe you want read access, so you could read what you wrote on Facebook. You could use the data for marketing, even if you just get his ID.
2. The user (and you) has to trust Facebook. They know on which page the user is logged on (you got a shop for dog food? the user will get ads for dog food as soon as he logs in!), and they could even log in as the user himself - theoretically, of course.
3. You are missing information like mail address and other things. There are workarounds, and they are working.
4. Don't ever (!) use only OpenID login or something similar (exception: you need to actually do things with the data you get - e.g. twitter bots)! You're forcing users to sign up for a social network they don't want. YouTube did this, and it wasn't very successful (except for the fact that there are "millions" of G+ users... Yeah.)

Except for that, I don't see anything wrong with OpenID login. Many big pages use it, and as long as the user has the possibility to log in conventionally, why shouldn't you use it?