the ultra-orthodox do, due to the birthrate. MO and traditional continue to decline. Reform is steady but not because reform jews aren't leaving, its cos they absorb ex orthodox jews (and new converts like me). Conversion has exploded. Like a lesbian member of my synagogue quipped, she grew up in an orthodox shul but if she had stayed, she would have had to leave her whole family at the door.
https://www.jpr.org.uk/reports/synagogue-membership-united-kingdom-2016
Critically, the sector that has declined most sharply is central Orthodoxy – broadly understood as the United Synagogue, the Federation and various independent modern Orthodox synagogues dotted around the country – which collectively have seen a 37% drop since 1990. This decline is partly due to disaffection, but it has also been driven considerably by natural decrease – more members dying than being born.
In contrast, membership of strictly Orthodox synagogues is growing. Indeed, it has grown dramatically over time – by 139% since 1990. A generation ago, the strictly Orthodox comprised 4.5% of all synagogue members households; today they comprise 13.5%. This growth is driven almost exclusively by demographic forces – particularly, high birth rates in this sector of the community.
Taken as a whole, Liberal, Reform and Masorti figures have been fairly stable over time. Liberal and Reform have both declined slightly since 1990, whereas Masorti has grown, albeit from a lower base. But this overall picture of stability is somewhat misleading: in reality, Liberal and Reform synagogues are both losing members at a similar rate to the central Orthodox ones, but unlike those central Orthodox ones, they are also attracting members from their religious ‘right’ to offset those losses.