If there is a non-compete clause in a contract you signed with the company, the agreement is between you and the company and enforceable as such--the company's relationships with other employees and contracts they may have signed with them is irrelevant.
Whether the restriction itself is valid depends on all the facts--reasonable scope, duration, etc.--which you would need to discuss with an attorney who can review your contract.
The company can enforce the non-compete agreement unless a court finds it is unreasonable as to time period, geographic area, and subject matter. It probably won't matter that they have not enforced it with every employee that found employment with competitors.
First, let me give a disclaimer that I am not an expert in non-competition agreements. That said, I think it is fair to say that just because your employer elected not to take action in earlier instances does not invalidate the agreement - it just means that they decided not to pursue it.
It may, however, speak to whether your employer has much faith in the enforceability of the agreements as written. There can be any number of reasons that they did not decide to sue on the earlier ones (including cost, time, resources, distraction from the business at hand, etc.) and you should not make the assumption that they will not pursue you under like circumstances. If you want more information about the enforceability of the agreement you signed, you should contant an employment law attorney that deals in this area.
This does not constitute legal advice. You should consult an attorney who can assess the facts of your case and discuss your legal rights and obligations.
I agree with the other attorneys advice. I would also like to add that the non-competes will probably be non-enforceable if you did not receive consideration for that contract when you signed it. In other words, did they give you something of value to get you to sign. This is a common failure in non-competes and could explain why they have not enforced it against others who left. Non-competes are not favored by the courts and will not be enforced unless they crossed all the t's and dotted all the i's, so to speak.