Extremely, iirc. According to wikipedia:

It is extremely contagious: nine out of ten people who are not immune and share living space with an infected person will be infected.[5] Furthermore, measles’s reproductive number estimates vary beyond the frequently cited range of 12 to 18,[17] with a 2017 review giving a range of 3.7 to 203.3

For context the reproductive number (average number of unexposed people a carrier will infect) of the most virulent strains of COVID-19 is 3-8. See: https://pubmed.ncbi.nlm.nih.gov/35262737/ (basic vs effective rate refers to the infectiveness in a naive population vs one which is taking measures and/or has immunity).

This is also all exponential so small increases in R number have big impacts nya.

Meta in particular has a specific record of social manipulation, which is why I think defederating them specifically is so important. Even if we collectively have mixed feelings on corporate instances in general, social media companies, especially those like Facebook, have a specific and direct record of manipulating people and the population nya. Facebook/Meta in particular, is probably the worst of any of them.