It's strange that you're seeing a slowdown caused by additional subscribers. The FiOS "nodes" in essence work in a sense where a single Fiber run from the CO is being split up to 64 ways. That doesn't mean that there are 64 people using the Fiber at once, but it's just what they could do. Typically the splits are 32 ways or less.
For you to be able to get 150Mbps/75Mbps service, you also have to be on GPON, which is 2.4Gbps down, 1.2Gbps up as a maximum for a single strand of Fiber, shared across the "Up to 64" split. At maximum utilization, assuming little overhead, the guaranteed bandwidth you should see under FULL saturation of that split is 30Mbps down. On the upload, 18Mbps.
I find it hard to imagine that all of your neighbors, if they all share the same split are all saturating their connections, causing the PON to also saturate. So with that said, it could be possible that the problem is at the OLT. Perhaps there's a lack of connectivity (ex: 1 Gig-E port) connecting the OLT to the core/backbone network. Or perhaps there's a problem with the Fiber someplace which requires a tech to come out and repair, which was aided by adding additional homes to a split, creating more variables for multi-mode fiber. Or perhaps the problem is with your Central office not having enough capacity in general. You can check this by going a few blocks over and finding people who are also seeing slow-downs.
But to start, definitely make Verizon aware, and have your neighbors who are also seeing slow speeds report in. Stress to your neighbors to hard wire to their routers and also perform tests with their Wireless card disabled, so there's less to blame. Avoid the upselling of course, but also test against both Verizon's own speed tests and external sites. Also talk them out of sending you a router. 90% of the cases I come across, the router is not the problem. If it is, the unit's probably extremely old and is dying.