When individuals are baptised as babies, by the promises their parents make and the graces that are given with the sacrament, technically speaking they are members of the christian community. When they become adults, they still have the graces but it is their decision whether not they wish to accept the promises made by their parents and the community when they were infants.
If they choose to ignore the promises and the graces associated with it, it is their choice. They never loose the Holy Spirit because the Holy Spirit is always there for the person and it respects the individual’s wishes. But every now and then, it will try to enter into the individual’s life when they least expect it. Sadly many choose to ignore these promises and fall away from the church. The causes are best left for another discussion, but when they disappear, it is important for the church to reach out to them and invite them home.
Unfortunately, the church rarely does this and they rely upon the grace of God to return them home when these individuals decide to get married or they have children and decide to return to the church and pass these sacraments to the next generation.
I think it is important for the church in general to look at their past church lists and invite them to return home. There are numerous methods this can be done but it begins with one person with a loving heart for Christ.
You mentioned you know many fallen away Catholics, the only way to invite them home is by individuals such as yourself inviting them to attend mass with you, or another fun activity at the parish. The reason why the evangelical churches are growing is because active Christians are asking their friends, coworkers and other people they meet to join them in church.
In essence, the Holy Spirit never leaves these people but it will use others such as yourself to bring them home into God’s arms. It is up to you to accept or refuse this challenge.