What informal networks or communication channels exist, and how can you effectively navigate them to build relationships?

One factor that impacts this is where you're located. In the headquarters, MPK, there are SO many employees and tons of food options in the bubble of the campus, we rarely went out.

But in Seattle or NYC or LA, there's probably more cohesion since each office is a bit smaller and there's more to do.

One easy way to become known as a new person is to plan a casual event on the weekend, e.g. pizza or coffee. Even if people don't show up, they'll remember you as the person who took the initiative to plan it!

...is it common for people to get team building starting after work with a pizza or a draft IPA in company like Meta?

Generally yes, but that culture has definitely dropped for several reasons:

1. The pandemic (duh)
2. The company has naturally gotten older as its engineers grew up. I remember the median age of a software engineer at Meta was as low as 24 or 25 in the early years. I imagine it's closer to 28-30 now
3. Meta, like most of FAANG, has shifted towards hiring far more senior folks, E5+ engineers. This produces the same effect as #2 where people being older generally means they have more responsibilities in their personal lives and it's therefore harder to hang out outside of work

Meta is one of the more social companies though (it's literally a social media company after all), so you should definitely attend as many outside team events as you can to build those relationships and try hosting them yourself if you have the time. Hanging out outside of work is one of the most straightforward ways to build up "extra credit" when it comes to job performance. Your #1 priority should be to have fun and be nice to others of course, but if you do this right, these deeper relationships will help you tremendously on the job too (a win-win!).

Check out this other thread for more inspiration: "How can I make the most out of a return to office?"