If I interview for staff at meta and I don’t get it will they consider me for senior?

Yes, you can still get an offer, but at a lower level. In fact, downleveling is quite common within Big Tech -- it happened to me at Google!

• This is common at Senior/Staff levels since they're both terminal levels. Someone with 15 YoE can come in as either level.
• This is less common for entry/mid-level roles since due to "up or out." If someone with 15 YoE passed the hiring bar for mid-level engineer but not senior, they'd get rejected.

The specific answer in your case to this depends on the macroeconomic condition and if you're being hired for a specific team/manager.

• If you're being hired for a very specific role that wants an E6 Staff Engineer, you'll get rejected if you don't meet the bar for that level.
• If you're part of pooled hiring, where you could join any number of teams at Meta, you likely have more flexibility with what team to join.

From my experience, Meta interviews are often done with +/-1 in mind, so you can go 1 higher or 1 lower. 1 higher won't really be relevant for an E6 loop though as the bar for E7 is insanely high.

There's additional nuance as Meta does have some years of experience targets. If you have 10 YOE and are in an E5 loop, it's not likely you can salvage your interview with an E4 offer (Meta will almost certainly just reject). I imagine it's the same case for E6 candidates if they have like 25+ YOE.

That being said, it's quite rare for someone to meet the Meta bar, just at a lower level than target (I saw this maybe 5% of the time). Candidates usually just fail outright.